I don't have an answer, but I would hope that it isn't a tremendous amount.
The difference between D+ and C+ should not be that great, in my opinion, because most players in real life probably wouldn't give that much more credibility to a team with a slightly better reputation.
Now if you're talking A+ versus B+, I would imagine that difference to be much greater than the one between C+ and D+ (i.e. a real life player might fall all over himself if Duke or NC came after him, but if someone a bit lower did, he might merely be satisfied or may not even like them at all, depending upon the player).