So my newest character found a blessed iron mace at lvl 3. I began using that enchantment when he gets petty soul gems and weapons from dungeon diving to sell. So the first time I enchant an iron dagger with it the value is like 688, the next one is 680. They both sold around 240. I've leveled up a bit and put perks in Enchanting and Speech. Now when I enchant them they're worth 450, and sell for 180 or so. Can anybody tell my why I'm getting less money for items now with higher Speech and Enchanting than I did before I invested in them?

It's a bug in the game that has yet to be fixed. See here, 4th item down: http://www.uesp.net/wiki/Skyrim:Enchanting#Bugs

Save and quit the game and then reload the save, it should fix the issue.

Note that on low levels then you only have like the fear or turn undead enchant and crappy skills you get more for the item than after an save and reload.