I have only recently gotten into TF2 trading, and I find it weird that keys are so low in value, when they can ONLY be purchased from the Man-Co store for $2.00. Were they a dropped item at one point or something?
It's actually $2.50 USD in Mann Co.. btw, if you haven't noticed, stuff in the TF2-conomy is much cheaper than prices on Steam, and prices in the Mann Co. Store.
Take the Fast Learner for example. It costs $10 USD in the store, yet is valued for 4 keys (at $1.50 each, it's worth $6) Thats +40% more than it's market price. Basically, once an item is out in the store, if you attempt to sell it at the same price, what's the point of buying it off of a stranger, when you can buy it more securely for the same price? So because of this disadvantage, sellers drop their prices a tad. Then a better offer appears. Prices drop yet again, then again, and again until it finally stabilizes.
Edit: Also forgot to add Steam games, when someone has at least a solid $10 bucks in their tf2 backpacks, most take advantage of that and use it to buy games for 40/50% its original value. In fact, weeks ago I got a steam for L4D2 for 4 keys + 1 Ref (1.50/key and 0.60/ref; $6.60) even though the game itself is sold for $20 on Steam. **Although I warn you, just like the prices of unusuals and such, these prices are very vague because of the fact that usually people sell games that they bought in a bundle, or while it was on sale** Again, L4D2 often goes on a weekend sale for $2 or so? And several games such as indie games get sold in bundles, such as the Humble Bundle and these bundles offer huge discounts on games. This places a price on games. Games that were originally bought, games that were gifted but unwanted, games on sale, or sold in a bundle for a nice price. Many people take advantages of these discounts and sell them individually for a profit. Of course, thousands if people do this andv thus the price of a game in tf2 item starts dropping. Others normally follow because they doubt they will sell for such high prices. Think of it this way: Just like dirty items, even though its unused, when you buy a game of someone, it sells just like a dirty item. Or you can compare to real life, like how people buy stuff at a really huge discounted price and sell it as a used item. for a bit of profitb of course.
That's why most people don't buy new (sexy) items such as the FL, the Nine-Pipe, the Aladdin's Reserve, etc when it first comes out. (What.. Why again?) It's because since there are few in exsistance, the price will obviously be pretty damn high. Once more start circulating in the market, the price drops. (Supply and demand)
So what's this have to do with keys? Well they are the same. Once more keys get poured into the market, the price will eventually start changing, and changing and changing. As of now, there are a lot of people who still buy it at the Mann Co. Store. However, it's not enough to change the price of keys.
Hope this helped. Quite long, but hopefully worth the read?
TL;DR.. TF2-Conomy, like many things work based on supply and demand at the beginning, then it's price stabilize and the rest of TF2 follows this price. **However, there are exceptions: Bills, Buds, Max's, basically the expensive promos.**