Big business. I can tell you from experience that athletic sponsorships go down to even the smaller schools. When I ran track in college, back in the eighties, our program was sponsored by Nike. I don't think the school received any money but we had a large room stocked with shoes for every sport. If we needed a pair of shoes the AD's secretary would give you the key to go grab whatever you need. Being a hurdler, triple jumper, long jumper, high jumper I had no problem getting shoes for each event. Heck the one time I ran the decathlon they took me in there to get the specialty shoes for each event. In addition to the shoes we received new uni's and practice clothes every other year.
I was was at one of the top D2 programs at the time. We competed D1 during the season and then dropped down to D2 for the national championships. 3 or the previous 5 years before I started there we won the national championship. We were a top 5 finisher one of my years and in the top 10 twice. From what I know the other D2 programs in our area had deals with nike or Adidas but it only included shoes, not as good as our deal.