Many vet clinics have standardised prescription fees, usually from $3 to $7 that help them to cover the cost of staff time in drawing up the prescription, entering it into the computer, eft pos fees, pill vials, labels, label printers, inventory control, drug logs and drug handling courses. If you had gotten 10 more tablets, it may not have cost more than a few additional cents!
I know it bites but as we know, veterinary medicines are not subsidised but they often cost the same as identical human medicines so clients have to bear the cost. To be perfectly honest, clinics that don't routinely charge prescription fees are probably mismanaged to some extent with their one or two vet nurses no doubt acting as janitors, kennel hands and receptionists instead of the skilled practitioners that they are! *off my soap box now*
I do feel for you though. The best thing to do would be to purchase a large bottle and share it with your friends (notice I did not say 'sell').