I've maintained a cemetery for many years and it seems to be you are fretting too much about the work needing to be done. Establish and hourly rate, give them a ballpark estimate. Do the work, fine tune your price and stick to it. Work is work, doesn't matter if it is a cemetery, church, whatever. I would not let them lowball you, if they don't want to pay then walk away. Simple as that. I'd jump on the job if I were you! Do a good job and you'll pick up more business from the church goers who see what a good job you do!