Not sure what your trip logistics are, but do you have to have a car at Cache Bar? If you are a passenger it would be way easy to get back to Stanley with personal gear and a couple of companies offer a shuttle from Boundary back to Stanley in the $60 to $65 range.
If you are looking to drop a boat and gear at Boundary and then drive to Cache and get a ride back. Make sure to plan ahead a bit as it is not always a given for sure. Again much easier to get back to Stanley than Boundary. And then maybe arrange to meet someone in your group to ride in. Anyone else finishing at Cache and who has room would likely not want to drive into Boundary and back out if they are on the way home.
A full shuttle of a car being picked up at Boundary and driven to Cache runs about $235 to $250 plus you need to leave $60 to $100 gas in the car so you have a full tank in Salmon before they drive in to Cache.
Good luck with everything. Enjoy the Middle Fork. She is waiting in awesomeness.