I bought a 125cc Atomik MotoX of ebay for 520.00 and its great.
I also bought a 250cc ProX of ebay for 1000.00 and thats great too
Personally i wouldnt buy a good brand bike for a couple of rides a month
I ride mine heaps, jump it, take it to the tracks and thrash it and it does fine.
Never stopped
Only problem i had was the front forks were too soft so i bought some fork oil and topped it up and now its perfect
Good luck with ur new bike!!