Depends on lots of things…. as mentioned above, but if your looking for a cheap(ish) & decent (new) cross country type hardtail, something along the line of the Carrera Fury, Decathlong Rockrider (8.1), or KHS Alite 2000 would be my starting point. Great value for money bikes without the 'brand name' to push up the price.
2nd hand bikes can offer more value for money but only if you have a good working knowledge of maintaining a bike, otherwise you could be looking @ replacing the drive-train and servicing the forks before too long, and not being any better off than you would buying new