There seems to be a lot of misinformation regarding this, including on the wikis. The price does not drop as your enchanting skill increases. I figured this out by gaining several skill points through disenchanting: none of my enchanted items lost value. They started losing value only after I created more items of the same enchantment. For example, if you enchant something with Turn Undead, all your weapons with that particular enchantment will lose value while all your apparel and weapons with a different enchantment will keep theirs. Additionally, this is done in steps, so you will not see a decrease after every enchantment.
TL; DR: When you flood the world with items of a particular enchantment, the price for all such items drops; all other items keep their value.
Edit: What further complicates things is that (as of 1.2) there's a bug upon loading a saved game. It looks like the prices of your items will not be updated and will remain at whatever they were before you loaded! To make the game recalculate and show the correct price for a class of items, it seems you need to make that particular enchantment again (other enchantments, changing item/character location, autotravel, etc. didn't seem to refresh the price).
And it's not only a display bug: you can exploit it by saving the game after you're done with your enchantments, loading a previous save, enchanting once in order to update prices, and loading your current save again to sell items at higher prices.
Bottom line: prices of enchanted items are completely broken and in certain cases you may wish to restart the whole game before loading.