Don't forget you'll need the whole wiring harness too, front to back, incl all the connectors for speakers, sub, amp, headunit. Would probably be easier to go straight to aftermarket... which is what I'm doing. Doesn't need to cost too much.
For your edumacation and comparison, this is what I'm putting in:
- Eclipse EA4200 4-chan amp (2 chans for front spkrs, 2-chan bridged for sub) $130 (Crutchfield, while stocks last)
- Kicker 3.5" (08KS350) spkrs for rear (still driven by headunit) $30
- Kicker CVT8 sub (replaced crappy stock unit) $70
- Alpine SPR-17S component spkrs for front (6.5" + tweeter + crossover) $136
You can spend more or less, but still beat the Monsoon system.