Need ideas for a fair way to add more teams to an auction draft keeper league.

We have a keeper league where we do a yearly auction draft. Everyone gets to pick a keeper each year by paying 10% more than they paid the year before. If you pick up a player mid-season from the waivers, you get to make that player your keeper by paying the ESPN recommended value based on the $200 leagues. Our league has a $500 budget, so it essentially comes out to picking a keeper at 40% of thier vaule if you found them on waivers during the season.

This year two additional teams want to join the league, and we are trying to find a way to make things fair in terms of keepers. One suggestion is to let them pick any player that did not end the season on someones rooster for 40% of their value (i.e., treat them as if they found the person on the ww during last season). Another suggestion is to tell the two teams wanting to enter the draft that they just don't get a keeper this year. Any other recommendations or ideas?