If you look at ebay there are units going for less than £20.
Don't know if they work with the NXT, or are as accurate as the dgps, but they are cheap!
Just go into ebay, enter 'gps (wireless,bluetooth)' and set the search to look for the lowest price inc p&p.
Update - Out of interest, I went back and looked for 'usb gps', found some units around £20 but they need an adaptor to fit the NXT and I have no idea if they are compatible.
Update - These were the cheapest I could see today - http://www.ebay.co.uk/itm/LATEST-SUPER- ... 519e0a7427
Don't know why they are described as for laptops, something to do with the aerial orientation? Or do they only work outside?