I'm in a similar situation... accepted to NSSR (Poli Sci MA), only 25% tuition waver, and thinking about the cost makes me want to vomit. I was told that their PhD funding is much better, although don't know how true that is. I've also been told that competition is fierce for the few PhD spots, so even if what I was told *is* true, it's far from a sure thing.
This reply isn't very helpful; I just wanted to express that I share your pain. My attitude going into the application process was that I only wanted to go to schools that were right for me, and consequently had no safety schools. If U. Miss doesn't entice you academically, screw it. I'm leaning toward eating the debt for the first year and working my ass off to qualify for better funding/scholarships the next.
Best of luck with your decision. If you accept NSSR's offer, maybe we can both save money by going in on a bulk case of ramen!