If you buy a subsidized Nexus One with T-Mobile and you cancel your contract after the 14-day trial, it'll cost you between $50 and $200. Sounds pretty standard, but Google will charge you an additional "equipment recovery fee."
Google has the right to charge you up to $350 if you cancel before the 4 month mark, which is essentially their way of thwarting any plans you may have to sell a former contract phone for profit.
But here's the catch: Google's fees plus T-Mobiles fees can add up to $550—which is $20 more than the Nexus One costs in the first place. Ugh, this math/cancellation is a bit confusing, but basically, Google will charge you whatever you didn't pay for a Nexus One due to subsidies. In other words, you'd owe Google $350 if you bought a Nexus One for $180 through T-Mobile—which brings the cost of your Nexus One up to $530 (what it runs unlocked).