Right now FedEx sends an A300 to Lubbock, then cross loads 4 Caravans to take freight to Abilene. Instead of all that, a CRJ could go straight to ABI from Memphis. UPS sends a MD-11 to DFW, then fills a Metro and a Braslia to fly to Midland. With a CRJ they could go straight from Louisville to the destination. Also some cities have heavys stop, unload half their freight, then fly onto their final destination half empty. Eliminating that stop or the second leg might make more sense.
I'm not sure a CRJ would be cheaper, but it's possible. It's not just the cost of the flight time, it's facilities and personel to sort and load at the smaller hubs, as well as the time lost. Esspecially if they could design a system for loading RJ sized "cans".
For on demand stuff, most of the work is done with turboprops, Lears, and Falcons. If you could put a bigger door on a 200 you would have a nice replacement for the Falcons. The purchase price could be offset by the savings in MX and fuel. A 700/900 would probably be too big for that work, and too small to replace the 727, 737, and Dc-9s.