Actually, the deal is better than that. Wilwood repairs your old MC and returns it to you, so you have the new MC and the old one for $100. Otherwise, the repair only is totally free.
"Wilwood will sell you a brand new master cylinder for $150. Once we have received your original recalled master cylinder for inspection and repair we will credit you $50 of this $150. Wilwood will still return your inspected and repaired master cylinder to you free of charge."