You can make large amounts of Gil in FFVII by selling on certain materia that you have mastered. The selling values vary from materia to materia, but you can earn over a million Gil just by selling one of the right type! To master a materia, you must accumulate enough AP to raise it to the maximum number of stars (visible in the Materia sub-menu).
Master Materia Easily
You can master materia faster by using weapons and armor that have "Double" or "Triple" growth rates rather than "Normal". To level up materia as quickly as possible, you will want to go Inside Northern Cave in Part 3 of the game. Follow the left path at the point where your party splits, and then take the up path. Here you encounter Magic Pots; Magic Pots will always demand Elixirs, so be prepared. You will want to use the "W-Item Cheat". Each Magic Pot you defeat will earn you 1000 AP. In the same area of the Northern Cave, you can encounter Movers. They always appear in a group of 3, and defeating all 3 will earn you 2400 AP.
Here's where it starts getting tricky. If you're good at math, read on. Otherwise, skip to the list below. Equip Cloud with the triple growth, 3 materia slot Apocalypse sword. 3 (triple growth) multiplied by 3 (number of materia slots) gives an efficiency of 9. Cid's Scimitar is interesting, as it also has triple growth, but in this case 3 (triple growth) multiplied by 2 (number of materia slots on the Scimitar) only gives you an efficiency of 6. Let's consider 1 (normal growth) multiplied by 8 (number of materia slots) - e.g. the Wizard Bracelet; or 2 (double growth) multiplied by 4 (number of materia slots) - e.g. the Rising Sun. Either of these 2 gives an efficiency of 8. An excellent option is Cid’s 5 materia slots double growth weapon, the Javelin, which gives the highest efficiency: 2 (double growth) multiplied by 5 (number of materia slots); 10. But let's say there were only 5 different materia you were interested in levelling up - in that case it would make most sense to equip the Apocalypse and the Scimitar, and get triple growth on each of those materia.
Buy in Sector 7 Slums, Sector 5 Slums, Wall Market, Fort Condor, Junon (Disc 1), Costa del Sol (Disc 1), Mideel. Find in No. 1 Reactor.
Buying Price 750 Gil
Selling Price (mastered) 52500 Gil
Heal
Level 1 | AP 0
Level 2 | AP 12000
Level 3 | AP 52000
Level 4 Mastered | AP 60000
Buy in Kalm, Junon, Costa del Sol (Disc 1), Gongaga.
Buying Price 1500 Gil
Selling Price (mastered) 105000 Gil
Revive
Level 1 | AP 0
Level 2 | AP 45000
Level 3 Mastered | AP 55000
Buy in Junon, Costa Del Sol (Disc 1).
Buying Price 3000 Gil
Selling Price (mastered) 210000 Gil
Full Cure
Level 1 | AP 0
Level 2 | AP 3000
Level 3 Mastered | AP 100000
Find in Cosmo Canyon (Disc 2 and 3).
Buying Price N/A
Selling Price (mastered) 1 Gil
Fire
Level 1 | AP 0
Level 2 | AP 2000
Level 3 | AP 18000
Level 4 Mastered | AP 35000
Red XIII's Initial Equipment.
Buy in Sector 7 Slums, Sector 5 Slums, Wall Market, Fort Condor, Junon (Disc 1), Costa del Sol, Mideel.
Buying Price 600 Gil
Selling Price (mastered) 42000 Gil
Ice
Level 1 | AP 0
Level 2 | AP 2000
Level 3 | AP 18000
Level 4 Mastered | AP 35000
Cloud's Initial Equipment. Buy in Sector 7 Slums, Sector 5 Slums, Wall Market, Fort Condor, Junon (Disc 1), Costa del Sol, Mideel.
Buying Price 600 Gil
Selling Price (mastered) 42000 Gil
Lightning
Level 1 | AP 0
Level 2 | AP 2000
Level 3 | AP 18000
Level 4 Mastered | AP 35000
Cloud's Initial Equipment. Buy in Sector 7 Slums, Sector 5 Slums, Wall Market, Fort Condor, Junon (Disc 1), Costa Del Sol, Mideel.
Buying Price 600 Gil
Selling Price (mastered) 42000 Gil
Poison
Level 1 | AP 0
Level 2 | AP 5000
Level 3 | AP 20000
Level 4 Mastered | AP 38000
Buy in Kalm, Junon (Disc 1), Costa Del Sol. Find in Shinra Building (Floor 67).
Buying Price 1500 Gil
Selling Price (mastered) 105000 Gil
Earth
Level 1 | AP 0
Level 2 | AP 6000
Level 3 | AP 22000
Level 4 Mastered | AP 40000
Buy in Kalm, Junon (Disc 1), Costa Del Sol.
Buying Price 1500 Gil
Selling Price (mastered) 105000 Gil
Gravity
Level 1 | AP 0
Level 2 | AP 10000
Level 3 | AP 20000
Level 4 Mastered | AP 40000
Buy in Costa Del Sol (Disc 2 and 3), Mideel (after Ultimate Weapon attacks). Find in Cave of the Gi.
Buying Price 8000 Gil
Selling Price (mastered) 560000 Gil
Comet
Level 1 | AP 0
Level 2 | AP 12000
Level 3 Mastered | AP 60000
Find in Forgotten Capital.
Buying Price N/A
Selling Price (mastered) 1400000 Gil
Contain
Level 1 | AP 0
Level 2 | AP 5000
Level 3 | AP 10000
Level 4 | AP 15000
Level 5 Mastered | AP 60000
Find in Mideel (feed the little white chocobo a Mimmett Greens, then scratch it behind its ear).
Buying Price N/A
Selling Price (mastered) 1 Gil
Ultima
Level 1 | AP 0
Level 2 | AP 5000
Level 3 Mastered | AP 100000
Find in North Corel (Disc 2 and 3, after train sequence. Received for free if you stop the train, buy for 50000 Gil if you crash the train, unobtainable if you never reach the train).
Buying Price N/A
Selling Price (mastered) 1 Gil
Seal
Level 1 | AP 0
Level 2 | AP 10000
Level 3 Mastered | AP 20000
Buy in Junon, Costa Del Sol (Disc 1).
Buying Price 3000 Gil
Selling Price (mastered) 210000 Gil
Transform
Level 1 | AP 0
Level 2 | AP 8000
Level 3 Mastered | AP 24000
Cait Sith's Initial Equipment
Buy in North Corel, Gongaga, Cosmo Canyon, Mideel (after Ultimate Weapon attacks). Find in Corel Bridge.
Buying Price 5000 Gil
Selling Price (mastered) 350000 Gil
Mystify
Level 1 | AP 0
Level 2 | AP 12000
Level 3 Mastered | AP 25000
Buy in Gongaga, Cosmo Canyon, Rocket Town.
Buying Price 6000 Gil
Selling Price (mastered) 420000 Gil
Exit
Level 1 | AP 0
Level 2 | AP 10000
Level 3 Mastered | AP 30000
Buy in Rocket Town.
Buying Price 10000 Gil
Selling Price (mastered) 700000 Gil
Time
Level 1 | AP 0
Level 2 | AP 8000
Level 3 | AP 20000
Level 4 Mastered | AP 42000
Buy in Gongaga, Rocket Town.
Buying Price 6000 Gil
Selling Price (mastered) 420000 Gil
Destruct
Level 1 | AP 0
Level 2 | AP 6000
Level 3 | AP 10000
Level 4 Mastered | AP 45000
Buy in Fort Condor (Disc 2 and 3), Mideel (after Ultimate Weapon attacks). Find in Shinra Mansion.
Buying Price 9000 Gil
Selling Price (mastered) 630000 Gil
Barrier
Level 1 | AP 0
Level 2 | AP 5000
Level 3 | AP 15000
Level 4 | AP 30000
Level 5 Mastered | AP 45000
Buy in Rocket Town.
Buying Price 10000 Gil
Selling Price (mastered) 700000 Gil
Shield
Level 1 | AP 0
Level 2 | AP 10000
Level 3 Mastered | AP 100000
Find in N. Cave.
Buying Price N/A
Selling Price (mastered) 1 Gil
Master Magic
Find in Kalm (trade Earth Harp, received for defeating Emerald Weapon), Cosmo Canyon (swap at Huge Materia for a mastered version of every other Magic Materia).
Find in Shinra Building, Junon Beginner's Hall, Forgotten Capital, Chocobo Sage's house (talk to green chocobo).
Buying Price N/A
Selling Price (mastered) 1 Gil
Master Command
Find in Kalm (trade Earth Harp, received for defeating Emerald Weapon), Cosmo Canyon (swap at yellow Huge Materia for a mastered version of each of Sense, Deathblow, Manipulate, Steal, Throw, Morph and Mime).
Red XIII's Initial Equipment. Buy in Fort Condor (Disc 2 and 3). Find in Sector 7 Slums, Shinra HQ Floor 63, Cargo Ship, Mt. Nibel, Great Glacier.
Buying Price 20000 Gil
Selling Price (mastered) 1400000 Gil
"All" materia sells for the joint highest amount of any mastered materia (1400000 Gil), and is the fastest to raise to mastered for a materia that sells for that amount. As such, it is effectively the materia of choice to level up when raising Gil.
Every other Support materia only sells for 1 Gil at mastered level.
