It can depend on what will happen to the pictures afterwards. If you standardise the frame size (with differing mattes inside probably) then you may be able to re-use them for your next show. If they are going to be sold from the exhibition then the framing should be good quality, or crap quality so the buyers pay re-framing costs directly of course?! There are probably very cheap plastic frames available somewhere, if they would be rigid enough for that size matte board.
All the cheaper framing options have very reflective glass of course. I recently found that a not-quite-right off the peg frame here was 52 Euros, decent custom made with cheap anti-reflective glass was 205 euros and then museum-glass was another 185 on top of that. I'm still wondering about that problem, as the print will be a gift!
There are alternatives to frames, giving less protection with no glass and no sealing. A triple thickness of foamboard as a backing, or thin foamboard as a barrier mounted on cut plywood, or your usual mattes and backing sandwiched between sheets of polycarbonate, or . . lots of things. As you asked about framing then I expect the alternatives are not going to make the right impact, or not give enough protection from prodding fingers and the like.