Jim,
we charge $75 for the collage plus the print price. At least that's our "listed" price for what I'd call "normal" collages. For complicated ones or ones where we have to find/create the background, the price can go as high as $150. However, the key will be what your market can bear in terms of pricing. These folks aren't used to spending a lot but a collage has more value than 3 separate prints.

Linda just did a double-truck center-spread collage of images from the ARC National for the magazine and that one must contain something like 25 images and it took her the better part of 1/2 a day to complete it. Something like that would obviously be a lot more expensive. I'll share it as soon as I get my hard drive back from her.

Jim