If you buy a subsidised Nexus One with T-Mobile and you cancel your contract after the 14-day trial, it'll cost you between $US50 and $US200. Sounds pretty standard, but Google will charge you an additional "equipment recovery fee".
Google has the right to charge you up to $US350 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. Basically, Google will charge you whatever you didn't pay for a Nexus One due to subsidies. In other words, you'd owe Google $US350 if you bought a Nexus One for $US180 through T-Mobile - which brings the cost of your Nexus One up to $US530 (what it runs unlocked).