How much does YC cost to attend (like boarding, food, transportation, social events)? Would those who attends likely come from a well off background?
The events YC provides are free. Depending on where you live your rent will obviously vary (you can pay $5000/month for an SF apt, or you can share a place in Mountain View for $600/month).
Like Zac said... the $$ invested should more than cover your expenses for the 3 months. Of course, that money is for kicking off your company - not fine dining.