A lot of it will depend on the economic makeup of the audience. Are they the type to bid high -sometimes even above value - because "it's for charity". Our school is 40+ percent economically disadvantaged families. We generallys saw more like the 50-60% of value.
Of course, the items make a big differenc also. For instance, I have a colleague who will also donate certificates for seats in a computer training class. The value is $200 each. But the average parent isn't going to pay nearly that. We'd usually get $20-$25 as a top bid.
Then there's always the fact that the "list price" value looks great on paper - but in reality you can always find certain items on sale so people won't really pay "list" for those.
The last two auctions I've been involved with netted 60% of value, average. Of course, two auctions is a pretty small sample and there are loads of variables that could affect your outcome, but that's our experience.
We have a number of items for our silent auction, and I've been keeping a tally of their total worth. From an auction, most items will get you something less than that. Is there any rule of thumb about what kind of profit percentage to expect? For example, if we have $5000 worth of stuff to auction, could we reasonably hope to net 80%, or some percentage (or percentage range)? Or are these things just too wildly unpredictable?