Go flat rate.  In our counties we offer a flat rate for the first 20-years of research and a per-year fee for each year therebefore.  Knowing that the county was established inb 1856, we kinow what the maximum research fee will be. 

As to copy costs, provide only digital images by digital camera, allowing you to pocket all profit fron the copies with the county getting no cut of this.  A flat rate per page at half of what the county would charge and capped at a reasonable point costs you the money in AA batteries and internet electrons to send images, at no other expense.   Saves the client money, nets you a reasonable added profit and stops feeding the counties unnecesary fees.  Win, win, win.