Chris brings up an interesting possibility:
Years ago, I did a gigantic job right out of photography school: $13K worth of food photography prints for a department store chains' cafeterias. To keep framing costs down, I mounted each photo onto Gaterfoam (Foam core board) to make each photo rigid (Shut up, Peter) and then mounted an undersized frame behind the photo to help it stand off from the wall and "float". In other words, with 30x40 prints, I made simple 1x2 frames that were, say, 26x36.
If you do, this, you avoid traditional matting & framing. You can also make the 1x2 "frames" yourself because they don't have to be perfect...they're behind the photos. Finally, and maybe best since we're talking collages, you can make some frames with 1x1s, some with 1x2s, some with 1x3s, etc., so that some photos "float" further out from the wall and can therefore overlap ones under it a bit.
A can of black paint comprises the "finishing" stage, and hanging them is easy!
I would have them professionally mounted though (shut up, Peter!).
