I guess the real question is why you want to cut them down. For some experimentals, cutting and repitching the blades was the only solution because there was no commercially available prop but, as noted above, it's a dangerous process. For the RV, there are a lot of commercially available props and little need to 'roll-yer-own'. Hartzell's 72" blended prop is actually a 74" prop cut down by the manufacturer; they've done all the testing and experimentation for you, so no need to take it farther.
Now, if you want to cut down a blade for repair, then you should probably take it to a shop for a couple of reasons. First, to evaluate the damage: if your tips are damaged, was it a prop strike and was there non-apparent damage as well. Second, the prop will need to be re-balanced. As for the cost, it'll probably depend on how much work is involved and what shop you choose. Apparently, the early responders like myself have focused on the safety issue; maybe other posters can make some shop recommendations.