Ok, let's look at this logically.
There are currently around 5,000,000 users on Steam. Let's say that out of those, only half of those cares about acheivements, and only half of those get all of them for one game. Assuming it's a $60 dollar game, and you get $6 back, that makes $208,000 loss for Valve. However, most people have more then one game. Most would have at least 5-10. Multiply that number by ten games, and you get $2,080,000 loss. now these are the bare-minimun numbers.Not to mention the fact that some games have less acheivements then others, and some have none at all. Black ops mutliplayer has 3 acheivements, only one of which takes any effort. (Oh I completely forgot about free-to-play games. That means that people could get money for free as well.)

In short, TANSTAAFL. There ain't no such thing as a free lunch. This will cost Valve a lot.

Of course the pre-step to implementing all of this is to declare achievement servers illegal otherwise Valve would be paying actual money to no good cheaters.

Then everybody would call his friend SAM for free money...