neither would I. If I could determine the approx. amount of the expected refund without doing the entire amended return, I would tell the client your refund looks like $50 but my fee is $XX which is going to be more than the $50 so it is up to you. Only if the amended would help or change future years - carryovers or something like that then I would say it should be done. If I can't quickly determine I say my starting price for amended returns is $XX no matter what the end result is. I still did the work. 

On a side note, you should have at least the min price figured out before starting the work and tell the client up front. Don't do the work, end up with $50 refund and they decide not to file and pay you for your time.

You actually did more work for this amended since you did not do the original returns. Tell them you have to do return wrong first then fix it. How much time did you spend and what are YOU worth.