I use a Lowepro Mini-Trekker for my RB67. I can hold the body with an attached lens (90mm) and back. Two more lenses (180 and hopefully someday I'll have a 65 or 55, right now I keep a holga where another lens could fit), probably 2 more backs, pentax spotmeter, filters, and a flash. Then I keep film in the outer section. It all fits nicely. And I think I bought the bag "used" from B&H (it was brand new, still had the tags on etc.) for a really good price like $60-75. And My tripod fits on the outside nicely.
-Grant