I'm debating on the same thing.. but going more for SQ rather than SPL. I'm probably going w/ two 8" subs (ID, RE, JL) behind the rear seats, about a 4" mounting depth for $100 a piece. For the doors I'm going to pop in the CDT ef-61's for $185 for the pair. I already have a Kicker KX600.1 amp for the subs from my old system, and a old school PPI for my highs.
2x 8" sub - $200
2x 6.5 comp speakers - $370
Total: $570
That leaves $430 for amps and mats if you need. Finding some good cheap amps won't be a problem for you. Specially if you run the rear speakers off you HU, and get a good 2chan amp for your front speakers. This will free up some cash for your sub amp.
Speaking of that, if anyone has a need for 2x Infinity Perfect 10.1's used for about a yr, shoot me a PM. They work fine and are in great condition. They are the 4ohm SVC. I also have 2x Infinity Perfects new in the box. They are 4ohm DVC. I need to get rid of them since they wont mount stealth in the ST. Let me know if you have a need for them.