Reenchant an already enchanted item

Click the item and press Charge - on PC it's default to T. You'll need a charged soul gem, though (the level of soul gem dictates the charge returned). This uses up the soul gem.

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:

1. 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.

2. 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.