I built my sorority's first composite. It cost us less than $120. We purchased our own film and asked my roommate photograph the sisters. I went to a framing store, and asked them to make a custom matte. It cost about $3 per hole, and $30 or so for the large frame. I organized the cropped photos, and they placed them in the holes. I think it turned out nicely for an amateur job.
The next year, someone else was in charge, but she fell through on her responsibility. Since then we have been using a professional company the charges way too much considering the quality of their work.