Don't forget the shipping which is the biggest expense on top of the rental fee. Then there is the cost to promote and market the exhibition, the staffing time and/or equipment for unloading the shipping crates and installing the exhibit - usually several people for several days and then the reverse at the end of the rental period, storage of the shipping crates if you do not have space on site, staff for monitoring the exhibition gallery and/or interpreting for visitors, special event for openings/receptions for donors/media preview, etc., maintenance and cleaning of exhibit components (this is, by nature of course, more of an issue in a children's museum), etc.
Depending on the exhibit title panels and labels are often included as part of the exhibit itself.
That's my quick, top of mind listing. Hope it helps.