I would say not a whole lot considering there are now OS 7 devices on the market and people can buy on or off contract. But I'd encourage you to check eBay, Craigslist/Kijiji to see what the 9780 is going for these days. See what an offer you can get
Bear in mind that a T-Mobile phone can only get 3G on the T-Mobile network so unless you buyer is happy with using EDGE and no 3G there was not a lot of point in unlocking this phone.
It really is only going to work to its full capacity on T-Mobile.