So if you'd rather not use the shoddy LP Store websites that usually have incorrect information. Here is the simple equation you can apply to find how much ISK per LP you will get out of the LP store.
Find open market value of item (buy orders) on https://evemarketer.com/ ~~ We'll call this variable G
Find ISK cost in LP store ~~ We'll call this variable D
Subtract D from G ~~ We'll call the answer variable H
Find the LP cost in LP Store ~~ We'll call this variable LP
Divide H by LP ~~ The answer to this is the ISK per Loyalty Point you will get on that item.
Hope that was easy enough to follow along. Cheers o/