Bullet proofing is a complex subject, and from your question, I suspect you haven't done your homework. First, there's no such thing as "bullet proof" - everything will be penetrated by larger or enough bullets. Do you want to stop a .22? a .30-06? a .50? a .50 with mk.211 rounds? anti-tank APFSDS? how many shots does it need to withstand?
The cheapest way for a home user would be to go to the scrapyard and pick up a piece of 1" steel plate. Heavy as hell, but will stop most rounds legal in america. But you really need to ask yourself what your goal is...
--Bushytails