I just don't think so. Here are the top 25 wins that Marshall has had in the last 3 years
Kansas
UNI
Creighton
Gonzaga
Ohio State
3 of those were in the tournament.
Don't get me wrong, he's a great coach, but that's not the resume of a $4million a year coach.
Based on that criteria, Marshall is closer to 4 million than Prohm 1.5 million.
How many top-25 wins Marshall had at a MVC program is a pretty poor metric of what he is worth to ISU. Any coach that improves the brand, changes how people view ISU, and is one of the safest bets to keep things rolling is worth $4 million imo. The act of hiring a guy like Marshall for $4 million has value in itself.