If you look hard enough you can find a used 80cc for pretty cheap. my uncle found my Ke100 for $100 but it needed a new top end ($200.00) so i get that bike for $300. And my friend found a nice RM80 for $600.00
It would be better to get a brand name bike. Sure an off brand is cheap but once it breaks its all down hill and it wouldent be worth it anymore.
I dont know any off brand bikes so i cant really help you there. But keep an eye open in the paper and cycle trader and you can find some pretty cheap bikes with monimal work eeded to them.