Well now, you could review the prices of other mods to get a basic idea but if you want the overall prices to be higher or use small change, then you can just scale them up or down.
I suggest that you have something that is a "base price" that all the other prices are calculated from, the least valuable thing in the game, such as dirt. You can decide on the prices by comparing the item's rarity to its usefulness. Naturally, diamond will be one of the highest things, with other ores scoring lower.
You could say that lapis is rare, but it's not very useful, so it scores below emeralds, which are a bit rarer and useful for trading. If you want to sell ores, you might need to think differently.
Just keep comparing them to each other and remember rarity and usefulness and you should be able to have something that works. Happy to have helped you.
