A lot depends on what you mean on keeping cost down. A cheap install to some people is a few grand, but to others is a couple hundred.
If you are into rock and not rap / hip hop I think 10's would be fine. I have one 12" and its plenty and I listen to both rock and rap. Plus I have the whole trunk dynomatted so no rattles. 8^)
I will tell you that RF punch are not the best, I have blown many of them in the past few years in my past cars. I even blew their HX2's. Make great amps, but not the best speakers, IMHO.
If you are looking for cheap , easy, and nothing fancy...I would get a pre-made box (JL Stealth, etc) or even a self powered Bass Tube.