How much is a signed Harry Potter book worth?

I have a copy of the Deathly Hallows, and on the front inner page it's signed by Daniel Radcliffe. On the back it's signed by Emma Watson, Rupert Grint, Michael Gambon (Dumbledore), Tom Felton (Draco) and I think 3 other actors from the films like Luna and others I forget, how much would the book be worth (especially as I have the main 3, dumbldore and Malfoys autograph on it). I got it signed at the Harry Potter Premiere on Thursday, it's authentic and the books in perfect condition...help would be great!