Collage pricing

[S-Man]

So I have set prices on my prints, and believe it or not, today is the first time someone has asked for a collage for a senior session. I don't know what to charge. I know it's going to take time to make the collage, but should I base it on a quantity of images used, or just mark up the size of the print a certain percentage?
Any help?
I did a search but didn't find anything specific to collages.

[kayumangi]

Never done one before but if it were me, I'd treat it like an album design and charge by the hour on top of the print costs.

Then you can use whatever formula you use to compute markup for prints.