I have promoted regular fund raisers and appeared in many myself.I have always employed pros and made sure they were paid.It might not have been their full regular fee, but they were working, they made a profit, added names to their mailing list, and sold cds. Peter's quote above is vital.."You are valued at what you charge". If the organisers and beneficiaries of the fund raiser don't have to pay the artists they invariably don't work their socks off to sell tickets, but if they have a target to reach before they make profit they DO work their socks off! If the artists wish to donate, that's up to them and a private matter. For most of the concerts I've arranged I've quoted a total fee for the night for the artists, but I have myself often worked for a guarantee against 50%, whichever is the greater, and have used my own mailing list and website to promote the gig alongside the efforts of the organisers. My own chosen charity is the hospice movement as well as fundraisers for local schools.
I know that the folk world has raised thousands and thousands of pounds for charity every year in Britain and a better way of helping ones fellows I don't know.
Oh, and by the way, the most frequently asked question after a concert is "Why don't we know these people, they're better than anything we see on TV!"