http://lukehutch.wordpress.com/2010/01/05/the-cheap-way-to-pay-for-a-nexus-one-think-tco/
In summary,
Total Cost of Ownership of buying the full priced Nexus 1 : $2173 (can switch anytime, can cut features out of the plan to make it cheaper)
TCO on buying the subsidized Nexus 1: $2292 (must stay on plan and contract for 2 years or face ETF)
edit: That article also brought to light that if you’ve been in a T-Mobile contract for at least a year, you can switch to the non-contract plan which is $20 cheaper by paying a $35 migration fee. Over the course of a year, that’s saving $20*12 – $35 = $205. I’m doing this tonight.
more details in this article:
http://androidandme.com/2009/10/carriers/t-mobile-news/new-rate-plans-for-t-mobile-start-today/
and
http://news.cnet.com/8301-17938_105-10382707-1.html