Can't recall the exact cost, but I purchased mine from the states and installed it myself when I had my Swan. Guess parts likely to be in region of $150 ish? Main cost would be if you need a heater or wanted to add more tanks or pump.
The install is fairly easy. Either choose same location as Jayco do or work out what's best for you. On my Swan, I used the free space behind the microwave rather than us up space in a cupboard. Measure several times, then made the initial cut only on the inside to check that there wasn't anything in the way that would cause a problem. Then drilled 4 corner holes before cutting the hole from the outside using these holes as a guide.
All the plumbing is via push fit 12mm John Guest parts that either buy from local caravan supplier, annual caravan show, or online such as caravanplus.com.au
What do you already have in terms of water heater, tanks, and pumps? Do you sent to use it when bush camping?