I'm not sure about the U.S, but here in the UK the Neptune comes in two varieties. Complete and ready to run, with a TX, RX, charger etc. and as a kit which includes everything except the Transmitter and receiver. If you have these already you will save yourself a few bob, which can go towards other things like a spare battery. Also it's worthwhile building it up yourself, as you'll learn how the boat goes together- subs are high maintenance vehicles, so it pays to know how things fit.
The good thing about the Neptune is that it goes together with a screwdriver- no glueing operations, filling- you don't even have to paint it, unless you want to.
There is also a lot of scope for modification in this vehicle.