Greg might be able to answer with specifics, but I think it's in the neighborhood of 150-200 for the average team, though it can be a pretty big range.
I have has as much as $280 I think, and as little as $40. That's over, gosh, 12 auctions now? (2001-2012. 2013 will be lucky 13.) But I think the average amount is in that 150-200 range, yeah.
Last season 483 players were kept going into the auction at a total cost of $5939, so roughly 20 players per team at $250 net salary each. 462 players were bought for $3189. These numbers may not be precise for the auction because my spreadsheet kept updating during the season, reflecting FA pickups and the release of some more expensive players.