use 120 film in 220 RB back?

[CGW]

Yes, to arrive at a 120 back from what you have now, all you need is the insert (and a way to protect the other insert from damage when you are not using it). A whole back may not be that much more expensive, though.

True. 120 Pro S backs usually aren't too pricey. They do often need their light seals replaced(usually the hinge and clasp seals are all that's bad)but it's an easy DIY fix.
120 inserts are kinda hard to find these days. Try to find one that's not had 100s of miles of film run thru it--telltale is missing paint off the advance lever, shell, and scratches all over the plastic of the insert.

[MattKing]

Unlike the Mamiya 645 backs and inserts, the RB backs and inserts are 90% the insert and 10% the rest. So I wouldn't expect inserts to be significantly cheaper than entire backs.

[hpulley]

You can still get Tri-X 320 220 rolls if you look hard.