As a photographer, it's generally expected that the use of your photos will represent some sort of credit back to you. However, rights to a photo or set of photos can be sold or licensed.

For their calendar, it sounds like they don't want the PHOTOS themselves containing the watermarks or credits. Verify with the customer, but it is typically acceptable to small print either under the photo with credits, or at least a sentence on the back giving credit. Of course, if they truly don't want any of that on the entire calendar, that would typically be offset by a small price increase (possibly in the 20% of final cost ball park).

As far as what to charge, that varies from person to person and shot to shot. If the shot is extremely unique, you could possibly offer it as a premium since they wouldn't be able to get it anywhere else. I think the bigger picture (no pun intended) here is if you add too much of a markup, they may not come back next year for another calendar. It would be a better business practice to offer the calendar at a slightly reduced final cost, in the hopes of more volume and future orders.