Why are my enchanted items worth less and less?
From UESP, under Bugs:
The value of enchanted equipment is inversely proportional to your current level of Enchanting. For example, a leather bracer enchanted while under the effects of a Fortify Enchanting potion will have less value than a leather bracer enchanted without a Fortify Enchanting potion. If creating enchanted items for sale, remove any Fortify Enchanting buffs you may have equipped and don't drink any Fortify Enchanting potions.
Assuming this isn't a glitch (it most likely is), it could just be a hidden mechanic to make creating the same item over and over again less profitable the more times you do it.
You can look at it one of two ways:
From a game design perspective, you wouldn't want players to be able to "farm" one item over and over again. This gets tedious and makes the game feel like a job.
From a role-playing perspective, when your first Turn Undead dagger was made, there were presumably only a few of them in the world of Skyrim. Now that you've made 86 of them, they aren't worth as much. You've turned a rare-ish magic weapon into something commonplace.
And before you shoot down #2 (because the first dagger you made is still worth more than the last one you made), it's obvious that the inhabitants of Skyrim will pay more for one of your early works to proudly display in their shops.
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.