Here is my personal rule of thumb:
1) I determine what I would find reasonable to pay for the item offered if I was buying it for non-charitable/helpful purposes.
2) I determine what I would find reasonable to kick in if it was not an auction but merely a cry for help for a situation I would find worth helping. I add this to the price of the item or items for my bids, sometimes starting low if it seems there might be multiple items of interest. If there are not multiple items of interest and no one else is bidding on something, I don't mind outbidding myself.
3) If there is some way to give to the cause outside the auction, I will do so in the amount of #2 if I lose all the auctions in which I have bid.
A lot of people are going through tough times right now, lots of them in ways I can't help, some in ways I can. Nobody has an unlimited amount of "hand" to lend. One thing I have right now, though, is the ability to make Guinness gingerbread and/or sour cherry bread. Which is, it turns out, probably better than nothing.