Refletive material for grow room

I am setting up a bedroom grow room. I'm on a very tight budget and was going to use foil for reflcting off the walls. Any feedback?

flat white paint would probally be the cheapest then there is
Mylar and Panda film which does get expensive
you could probally paint the room for 15-20 bucks

You could cover the walls with these thin emergency blankets aka mylar. You will find it in the camping supplies section at WalMart for like $3