I was talking to Grant at RAD motorcycles in Northampton about this not long ago. He thought I'd get about £1200 - 1500 if I tried to PX it and I might scrape £2000 if I sold it privately. This is not because of any faults on the bike but because it's done nearly 50K miles, and most people are petrified of "high-milage" bikes.
As he said, why would I sell it for that? The engine is great, it handles well and it's in good cosmetic condition. I've not seen anything more modern that makes me want to change.
So, if you've got a Fazer that has sat in a garage, wrapped in cotton wool, and has low miles, you might get £3000+, but if you use the bike as intended, don't expect to get much for it.