You need to cover your material costs, your printer and computer overheads, your office overheads and your time.
If you have to work on short deadlines, and you are printing when you want to do something else, your rate might be different to what you would charge if it was work you could fit in at your convenience.
If you have to do test prints for approval, cost for it... if you have a well-profiled system and can expect to get it right first time...
Are the pictures consistent (e.g. studio) and do they come ready re-touched and colour-sorted or do you have to work on them?
You need to consider your personal time and machine time - if you do long print runs It takes less of your time, more machine time.
In euro-ling, I was thinking of about £1/A4 for ink. (A4 is 297 mm * 220 mm, or about 2/3 of a square foot)) Epsom premium semi-gloss is £100/30M*22" or £3.33/24*36" print.
Ink is a significant part of the cost, and the currently popular large underexposed areas use more Ink.
24*36" is about A1, which is 8*A4, so, on that basis, material cost would be £11.33/A1 or 24 *30".
I would hope that the life expectancy of the printer would be in excess of 10,000 prints, so at a cost of £10,000 (including Photoshop, RIP, printer and computer), £1 per print might cover your hardware overheads.
Add about the same for office/studio overheads (or four times as much for NY, or city centre). Will they give you enough work to cover your overheads?
The personal time depends on the number of prints more than the area. Roll paper is the obvious option, particularly if you do not have to change rolls ( or use sheets) for different sizes, but will you have to change rolls for different papers?
Have they come to you because they know the quality that the 7900 can produce? ... Or are they just hoping that you will not be aware of the true costs?
Will they collect from you, or do you have to deliver or post and pack?
If the pictures are very hi-res will this slow you down? Do you need a faster computer/card reader ...
Are you a one man band?
If they do not want to pay for quality - price yourself out of the job and let them use the local commercial printer - what do they charge?