What kind of riding will you do?  How often?  I go infrequently - sometimes with my kids - and the lower end bike was fine (around $260 - Specialized Hardrock).  I do fairly easy stuff, some moderate single track, lots of gravel roads.  St Edwards Park, Lord Hill, Iron Horse trail, Mackworth park, etc. 
  
I recommend getting a bike with the shock absorber on the front - but that's just one step above minimal.   
  
I don't have toe clips like I do for my road bike.  That'll cost you a bit more.