I fitted an aquamist kit to my last car, a MR2 Turbo.
Very easy to fit, kit even included a tap to thread the hole you have to drill.
The advice i was given from the guy selling them was to get the nozzle as close to the inlet butterflys as possible as if it is put just after the intercooler you lost a lot of the cooling effect.
I also fitted a washer bottle with a low level warning light (got that from a scrapyard) so i would know when to water level was low and not drive to silly.
You may have to get the car on a rolling road afterward, i certainly did on the MR2 as it ran like cack after fitting, once it had been set up it drove lovely.
Dunno what the total cost was, kit was about Â£300 then the set up on the RR, probably about Â£400 in total.
hope this helps