Don't forget to add the cost of wiring to your project. You'll have 3 amps in this system so you'll want a fused dist block and good heavy gauge wiring. Also, plan your mounting location and what you'll need to secure the amps.
This is a good start.
Down the road do you plan to go bigger? Adding second battery to run sterio, perko switch or equivelint, EQ or source to control sub volume and fade between tower and inboat speakers?
Add pics when you can.