As it happens, I played a funeral this morning, and I've no idea if someone will drop me some cash later, and I don't really care. I did it for a friend and I've asked for nothing so I expect nothing. My experience though is that most often people will say thank you with money. Sometimes I accept in with grace, and sometimes I will donate it back to a cause. Rich people and strangers are given a price up front which varies according to my preparation work and inconvenience. No lower than $50 Cdn. and generally much more. I'm sometimes uncomfortable accepting money from friends even though it makes them feel better to pay, but their feelings are more important than my own in this regard.I'm aware of many musicians who insist on the payment up front because they've been stiffed or shorted for church ceremonies in the past. A regrettable situation when honesty should abound.
As I've got older I've also turned down such gigs which I'm sure would only piss me off, but I've only my conscience to deal with for that.
The short answer is to find what the market will bear and charge accordingly. Professional performance deserves professional payment.
Mooh.