I'm not as worried about money as I am other things...
He's paid well, has full support of AD, getting facilities upgrades, can win 7 games a year and keep his job, etc. There are plenty of things that should keep him happy in Starkville and make us comfortable that he's staying.
However, the guy is a fierce competitor and if he thinks he has an easier road recruiting and path to championships, I could easily see him jumping ship to [insert traditional power here]. That's not to say I don't think he can do it here, but if he's ever frustrated by the talent pipeline, I could see him bolting.
Does that happen this year when a lot of great programs are going to be searching and Mullen is likely finishing up his best season at MSU? I don't know, but with that combo, he's probably going to have a lot to think about when the season is wrapped up. The good news is that it has to be much tougher for him to leave knowing the projects he helped start (practice facilities, stadium expansion) aren't finished yet. He hasn't gotten close to hitting a ceiling yet at MSU, so that may keep him around if some great programs try to lure him away this year. It's a risk/reward scenario at that point. Building things into a success are more satisfying to leaders than taking over something that's been proven to work. The risk is that you can tarnish your name if you fail in the process. We shall see...
Other important questions I've never seen answered: Does his contract have a big buyout? Are there clauses that prevent him from leaving to certain schools?