I just went through this process for the first time and posted about it. The link to the thread is below, but here are some key takeaways:
1) For an item that is $1500-$2000 that does not require much work (ie. it is already authenticated; it does not require them to do additional research etc) there is no reason not to ask them to wave the consignment fee. They are doing almost no work with the item and taking on no risk. It doesn't matter that you have little money in it. They don't give you more money if you have a lot of money in it. Simply tell them that you think they are an elite auction house but you are speaking to a couple of others and given the ease of listing the item, are looking for one who is willing to work with you on this.
2) If there are "non-obvious" factors about the baseball, make sure to provide them for them. I won a baseball used in the 1945 Navy All-Star
Game. It also happened to be signed by Wimpy Quinn, one of the toughest signers of the 40's. This latter detail was not mentioned.
I also won a Negro League signed baseball referred to as "Jackie Robinson and others." Some research tied all the players to the 1953 Barnstorming Tour that he did.
Make sure you've done your research and share that info with them. Don't assume they will do it at the level you want.
3) Consider if the item will sell as well on eBay. For many cards, eBay provides a similar sale price but is quicker and has lower fees. You can list yourself or have a consignor list on eBay.
4) Make sure you are comfortable with the timetable. Auction houses often need the item a decent amount of time in advance to the auction, and it may take a few weeks for payment after the fact.
Here is the link to the thread I started:
http://www.net54baseball.com/showthread.php?t=240100