I'd have a rate for additional 1 - 4 then 5 - 10, then 10 and over - that way you can scale down depending on numbers.  for 1 - 5 $100 may be too cheap, but for over 10, if they are geographically close then you could drop to $80 per.  Depends on what else you could be doing and how long they take.  Remember there is the time to test, and you need to build in the admin time to do the certificate as well, cost of printing it off too...  Otherwise sounds pretty good.