Where’s a good place to mine a solid amount of iron ore

Like Cloudancer said, one of the first mines in the game is to the east of the Guardian Stones. If you fast travel there, you can look for the "Mine" icon in your compass:

Follow that, and it should lead you directly to the Embershard Mine. Once inside, you'll need to look for veins of ore. Look for any sort of discoloration in the walls, then walk up and see if the tooltip says that you're looking at ore. Here's what an Iron Ore vein can look like:

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.