I would suggest you could come up with a fair price by charging by the hours required per chain x whatever hourly rate you believe covers your personal time and facility/machinery/consumables.

For example if sharpening a chain takes you 20 minutes (1/3 hr) and you figure your time/space/consumables total $21 an operating hour then $7. per chain is a fair price. If it only takes you 10 minutes on a small chain then 10 min = 1/6 hr x $21 = $3.50 is a fair price. This way it would take into account the increased time for a larger and longer chain vs a small one. 

I always appreciate a sharpening job where the operator doesn't wear down my chain excessively in the sharpening process. I have taken in some chains  for first time sharpening and they come back sharp but about worn out. I don't mind paying a bit more for a sharpening job where the chain is sharpened but not worn out in the process.

Good Luck.

Jim at Dawn