We're in somewhat similar boats, you and I. I'm debating between Pitt and AU, but I did apply to DU. For my purposes, DU and AU would have cost about the same, so it was easy to cut out Denver. I think we're looking at about the same cost differences between the schools though. Same amount for Pitt, and my AU cost roughly the same as your DU.
Some of the advice I received from a polisci professor is that if your plan is to go into academia, what school you start off at might not make that big of a difference (assuming you do an MA now, and then follow up with a PhD later). If you're trying to get into a policy career, the same source said that prestige and location would be the priorities. Pitt is close enough to D.C. that you could drive there, but I wouldn't realistically expect to make that trip often while you're taking classes. (It's maybe a 4hr trip one way, but that gets old fast). But it's a lot easier and cheaper to get there than from Denver. Pitt has a semester in D.C. program that might make up some of the difference, but I understand DU is also part of it.
I liked DU when I looked at it, but even after 20k/yr in financial aid it felt overpriced (again, comparing it to AU). They do a great job with marketing, but remember to focus on the actual cost without getting hung up on the aid amount. I don't know anything about UCSD, and I'm mostly coming from a regional perspective, but I get the impression that it becomes more relevant to stay on the west coast if you're interested in, say, Asian affairs. I'd tend to agree that quantitative skills are important, but for better or worse, networking is still important in international affairs so there is a benefit to being close to the area you and others want to work in. It's a tricky decision to be sure.
Did you make it to the open house at Pitt? Academically, what are you trying to focus on?