I tried to reproduce on my own game i found something VERY interesting too !
Just edit the first dword "1" value you find to dword "3" will drop the item cost to free, no need to look for float value at the end, edit the first dword to "2" and the item will cost gems. As you can see on the second picture, i can even set a discount on a free item
If you lock the material ammount value, you can [buy > close shop > open shop > buy] items a indefinitely for free.
After being bought, the item ammount value turn to "105 354" (basically 66535 without xor encoding, this value is the word maximum range value). Edit this one to any available value will make the item came back in the shop, this is why lock the item value will let item in the shop.