I would estimate how long it would take to complete the job and apply an hourly rate.
So if you think it'll take 3 hours, and you charge $40 per hour, it'll be $120.
The hard part in a graveyard would be to work out how long it would take since there are so many obstacles. Unless you've done it before it's a bit hard to estimate it accurately.
Has anybody else been working that graveyard? If so, can you find out how long it took them to do the job? That would help with your estimate a lot.
You don't want to be stuck with a job that you seriously underestimated!