A few people have asked me to analyze exactly what disenchanting I do and how I go about it. I am hesitant to give explicit numbers since no two servers are exactly the same. I have a friend who I may be going into business with and because of that and my desire to turn this experience into some sort of academic project I have an excel sheet of disenchanting guidelines I use. The prices below are in gold. This is all the juicy stuff:
| Item Level | Max Price | Expected Value | Minimum Profit |
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| 36-40 Armor | 2 | 4 | 2 |
| 36-40 Weapon | 4 | 6 | 2 |
| 41-45 Armor | 2 | 4 | 2 |
| 41-45 Weapon | 5 | 10 | 5 |
| 46-50 Armor | 2 | 3 | 1 |
| 46-50 Weapon | 5 | 8 | 3 |
| 51-56 Armor | 4 | 14 | 10 |
| 51-56 Weapon | 10 | 17 | 7 |
| 64-70 Armor | 4 | 6 | 2 |
| 64-70 Weapon | 7 | 11 | 4 |
| 71-75 Armor | 11 | 16 | 5 |
| 71-75 Weapon | 11 | 16 | 5 |
| 76-80 Armor | 20 | 40 | 20 |
| 76-80 Weapon | 20 | 40 | 20 |
As a general rule the value of the materials your piece of armor or weapon will disenchant to changes every 5 levels or so. There are significant caveats to this analysis, but for now lets just look at the chart. I don't fiddle with anything below level 36, because in most cases the profit is less than 1g. The max price is obviously the most I will pay for the item. The expected value is approximately how much the piece is worth after it has been disenchanted. I always round downward to be safe, and because there are more items than I have time to buy. Minimum profit is the least I expect to make from the item. These values are on my server based on the enchantrix price from scanning the auction house regularly.
To access the different items what I do is go to the browse window in the auctioneer advanced and specify the level range I want to work with and change the rarity to uncommon. You also want to arrange the listings by bid price. I bid on all items that meet my requirements too. Just because the buyout price it out of my range doesn't mean the bid price is. I have plenty of time. I don't mind waiting for it.
As you can see from the chart the new wrath items are the most profitable. However those numbers represent aggregate expected value for the level range. Sometimes you get better stuff when you disenchant than others. I assure you, it balances out.
Some general advice though, stay away from non-wrath blues. Strangely enough they are usually worth less than the greens and cost more.
This gets even more complicated however. In addition to level requirements, armor/weaons have ilvls. Where the ilvl changes sometimes can be very valuable in wrath. When you search for items from 64-70 you'll see some items with ilvl 130 and up. Those are wrath items which are worth considerably more than BC items. There is another switch at ilvl 151. Items at that ilvl are worth around 40g a piece and can sell for as little as 5g.
The problem is that disenchanting takes time. For me it is no big deal. I buy all the items with one character, send them to another and read a book while I disenchant. I can also do it on my laptop at work. Since it only gets 10fps when I move, it's the perfect activity.
I will be adding screenshots and more information to this post later. Feel free to ask further specific questions in the comments and I'll try to update this post to address them. I'd like to make this post as complete and as valuable as possible.
Update 12/8
Prices for Northrend mats have cooled down a bit in the auction house. Remember to always use your data. My data is only useful to me and other players on my server. Hopefully, none of those people are reading this. What follows is a screenshot of me using auctioneer advanced and the information that is posted there. I know some people are still having trouble with the new version. If you would get used to the newfangled layout, I promise you'd like it better. You need to click to enlarge it to get any detail.

