It breaks down like this after many updates.
If you are simply updating your data plan the price is $379 for the phone.
If you are adding a data plan the price is $279 for the phone.
If you are a new T-Mobile customer, the price is $180.
This appears to be an issue between HTC and T-Mobile with subsidies and such. You can read the full article here: Only New T-Mobile Customers See Full Nexus One Discount - GooglePhone - Gizmodo