short answer:
one cheap enough that it can stand being used by anything and everything an 11 year old boy is going to do to it, yet not be so cheap as to endanger his life, AND not be so expensive that in 4 or 5 years, when he's outgrown it, you can't buy him a better bike.
so, don't buy a cheap department store bike, look for a semi-quality used bike. talk to a local bike shop and see if anybody there has any old kids bikes they can sell privately?