Hi Elmer:
As you probably know, custom milling fees vary as some mills charge by the job, by the hour, per board foot, or even on barter.
What you've shared with me is definitely more of a barter agreement where they've offered to custom mill and provide you with the lumber from your two smallest walnut logs in exchange for your two largest walnut logs plus $200 cash.
Estimated board feet from the two largest logs using International 1/4 scale:
Log 1: 20" x 20' = 350 board feet Log 2: 20" x 17' = 232 board feet
To determine if this is a fair deal, ask them what their hourly rate and per board foot rate is and do the math to determine if their original offer sounds reasonable.
Best of luck!