Over the past month to two months the price of keys has been moving towards equilibrium. There are a few reasons but perhaps the best answer as to why is the underground resale price being around $2 per key. If you can buy a key legitimately and risk-free for $2.49 and use it right away (although trading requires the 2 day waiting period) then buying one on the black market (more risk) should cost less, just as it does.
Although that does not explain the seemingly "settled" price of around $2 per key. Our best estimate is that keys are tied to refined metal in a 2:1 ratio in people's heads now (as are buds/bills). The price has to be near the official price but why not $2.25 or closer? The market has decided that for now the risk of buying a black market key is about $0.50 per key, that's it. If there was no waiting period, the value of an illicit key would be less as it was when there was no waiting period (remember those $1.65 key days on source op?).
More recently, the variance in the refined:key ratio has been getting narrower as well. The buy for 2 refined or sale for 2.33 refined are not unheard of but most often keys trade for 2.11 or 2.22 refined now. That is an efficient market and the sign the influx of the F2P players have made their mark. Some people complained about the new players, I say bring them on. They have reinvigorated the TF2 item market. With so many buyers and sellers keys trade at a more pinpointed price.
Additional thoughts welcome.