I can understand the salary differential
We had to pay up for Geno's services based upon his proven pedigree ie. 2 time Mac coach of the year, 2 Mac regular season championships ,and success in recruiting.
When Jim was offered the job he probably came in at a salary level which was commensurate with an unproven college coaching career, unlike Geno. He did have a stellar reputation as a player both from a college and professional standpoint