Wow, great work figuring this out !
On the PSX version, this data is located in the SHOPMENU.MNU file. Your topic gave me the urge to do some research, and I figured out where the prices for items are located.
In this PSX file, the items' prices are located at offset 0x6854.
The price for Potion is there and is comprised of two bytes : 32 00 (50 gil). Then, two bytes after this one (I don't know what those 2 bytes are used for, they are always 00 00), there is the price for the next item (Hi-Potion) : 2C 01 (300 gils). Then, we have 00 00 again, 02 00 (the price for the X-Potions), 00 00, DC 05 for the price of Ethers, etc...
If you want to find out at which offset in the PC version (in FF7.exe) this data is stored, just search for these bytes in your hex editor :
32 00 00 00 2C 01 00 00 02 00 00 00 DC 05 00 00 02 00 00 00 02 00 00 00 02 00 00 00 ...
The items are listed in the same order as in the kernel.
On a side note, could you give me a list of bytes in the area where you found the ALL materia's cost, LKM ? It would allow me to search for them in the SHOPMENU.MNU file

EDIT : Never mind about me asking about a series of bytes to find the materia's prices. I found it. After the items, there are a bunch of 00s and then there are the weapons' prices (starting with Cloud's, etc). Maybe the armor's prices are in there too. Then there are a bunch of FFs and we get the materia's prices - once again in the kernel's order (meaning that it starts with MP Plus, HP Plus, Speed Plus, Magic Plus, etc).
What I would like to find though, is the byte responsible for a mastered materia's price. The price for a mastered materia is 70 x its initial price, and it's way too high IMO.