They had to search and scan the pics for me and that cost if I recall correctly 28€ per pic/scan. I think it took them quite awhile and it's not a volunteer place but paid staff.
PS: It might be cheaper or even for free if you go to Frankfurt and take pics of the pics yourself. That was the case with the archive in Hannover, and I think Frankfurt told me the same, but it was too far for me to got there (for less than I paid).