Boxes on ebay with specs for these subs will be about 50-60 shipped. You will also need an amp to power them, a hi-lo convertor, plus a wiring kit. All in all, a very easy install. Probably a 5 on a scale of 1-10 with 10 being hardest. 1 being blacklines.
This is a very easy cheap mod for great sound. Good bang for your buck if you find good prices and you don't need to spend lots of money for top of the line, brand-new model equipment.