Scenario: I'm at a fundraiser and I place a winning bid on a piece of art. Is the amout of my bid deductable or is considered a purchase?

It's a purchase. When you receive something of value in exchange for your payment at a fund raiser, the charitable donation is the difference between what you paid and the value of good and services received. In this case, the value of the painting that you won is equal to what you bid for it, as the bidding process at an open auction is an excellent way of determining an item's fair market value.

Now here's a different sceanrio: if you were to bid $100 for a car wash being run by your church's youth group, you can make the case that the fair value of the wash is only $5 (assuming that's the prevailing price at the local commercial car wash in your area), and you have made a $95 charitable donaton.