How long have you been doing it? If you just started, I would just simply tell them..."Look, due to this ground being as rough as it is, and with all the oblects and such, I'm sorry, but I just cannot continue to service your lawnnfor $35. It would take a minimum of $50, and if you choose to try to find someone else to do it, I fully understand."
Just be honest, open, and upfront. They will understand, and it either gets you off the hook, or it gets youthe ompensation needed to make up for the extra time and care required to do this job at a profitable rate.
Thank you, Dad - for always being the dad that you were. You truly are my hero. You always were.