Yeah, me and Kalanyr are trying to make an enchantments list/price system that makes sense throughout the game. Here is my original excel file. Hp/MP is Value to the 1.75th power, and Strength/Defense is to the 2nd.
Kalanyr's idea of using squared on both and just reducing HP/MP to 75% might work pretty well too. I've got some interesting items in the attachment, in my opinion. Ignore the stuff after the 3 line break though, that's just there for testing higher end values to see if the mechanics remain stable out to rediculous levels (which they almost never will).