IMO you shouldnt need more than 1,000 feet I think that is beyond the distance you can see you vehicle
I havnt done it myself but I think its under $150
the mod is half way down the page it was done by a drag guy
I have seen those guys use them. The range is greatly improved over stock, but it's used for speedruns. Dragracing only requires 300 feet range, and of that the first 132 feet is used for the actual dragrace.
The DX3R is a great choice, thats what i'm going to get. I know a guy that has one and we used the "traxxas" method (two way radios, one guy carries the truck and one guy carries the radio the opposite way until the guy with the truck says it is no longer responding to tx inputs) to check the range, it was good for just over 4100 feet. Keep in mind this was over open pasture though.