I like TomB's suggestion, use this as a learning experience. Go, look at the coins, determine what you think they are worth, and watch how they actually do in the auction. Typically most of the stuff at one of these sales is common material basically worth melt. There may be a few better items but be careful many of those will be problem items. Then don't be surprised at the sale to see the common silver going for twice melt, common circulated silver dollars that should be around $28 to $30 bringing fifty or more, and the problem coins sell like they are problem free. Watch and learn.
Those would be considered absurd premiums from an auctioneer in Minnesota and would likely get him run out of town REAL fast in the smaller communities. When one of our auctioneers had the audacity to start charging a 5 percent buyers premium I thought they were going to lynch him. His basis for it was well justified as well, having just purchased a new building in a VERY small town for the sole purpose of holding auctions there rather than on site, where he and his customers had to deal with the elements of Minnesota winter. We see anywhere from 0-10 percent buyers premiums and sometimes another 2 percent taxed on for credit cards. The auctioneer is expected to make the majority of his money from the seller(s).