I assume this is a keeper league. If this is a first year league there are several sources for $ values but most you will need to adjust because they are based on 12 team leagues. There is more money to spend in an 18 team league and more players will be bought. Your best bet is probably rototimes because you can input your league settings including the stats you will use, the number of teams, and the number of players per team.
If this is an existing league (not the first year), you pretty much have to make your own $ values. Most players that teams keep will be undervalued meaning there is more money to spend on the remaining players. In this case it is best to make a spreadsheet of the available players, determine how many at each position will be needed to fill out teams, add up the money that each team has left to spend to find a total amount for the league that will be spent at the auction, and then assign dollar values accordingly.