What to charge for a permanent exhibition

I work in human resources for a large lawyers office in the Netherlands. They asked me if I would let them know how much I would charge them per month if I would exhibit 15 of my pictures (framed)and change them every half year. I really don't know what to charge, so please I need some advice.

Take a hard look at your costs, doing the prints, mats, frames, etc. Your charge should cover all of that plus your time. Since you get them back, you can sell them and make money from them, so that also can be considered. A number that comes to mind to me is $5 -10 a month per photo. Over 6 month is would cover your cost if you justwant to do this without loosing money.
If they have a real budget for it, I would probably charge the price of one full piece a month, and after 2 years, they can keep one full set of their choosing if they decide to stop rotating prints.

Thanks Rob, I was thinking about something in that range. I think they do have a real budget for things like this. Your reply really helped making my decision. Thanks.