UT Dallas and Sky mates at Dallas(if there's something a lot cheaper PLEASE Let me know!)
Factor in the hourly difference in rates - few actually get their ratings in the minimum time. The difference could be closer to 10k. Training in a big metro area is usually more expensive than a smaller school at an outlying field. Quality will depend on the individual CFI you work with, and you've got no better odds at a big school than a small one. Ask around ...
Oh, and I almost forgot. Interview with Mesa? Nice. I see Mesa has had some input into the program. What's important is how many are
hired, and how many are hired from other programs. Do you
really have a competitive advantage attending one school over another? I don't know. You need to find out.
If your heart is set on flying, the second major would be a waste of time. As a backup it could help you, and might count towards an advanced degree if aviation doesn't work out and you go back to school. It seems you're having your cake and eating it, too - a non-aviation degree as a back up while enrolled in an aviation program. It's a good idea, but it's also a HUGE amount of work to be worthwhile. Double major in something that will impress a prospective (non-aviation) employer. Anything with "golf" in it won't do the trick, frankly. If you've got the time and smarts, do something scientific or technical. If that's too much, find something that dovetails with your aviation management classes - perhaps a business degree. However, if the second degree is really worthwhile I doubt you can do it in four years. Now we're back to just getting the technical degree alone (in four years or a little more) and flying on the side - which suddenly looks a lot more cost-effective than a five year (or more) double major at an aviation program school.
Either way may work for you, and there's a lot more involved than money - the more expensive way may be more in tune with your plans, likes, and dislikes. A lot of good people go to Purdue, Riddle, etc. and do fine. It's not necessarily something you can analyze on a spreadsheet. Stop and think. You will be taking real courses, in real time , with real professors. Some of the courses will be harder than hell. With two majors, most of them will - and you are flying too. How long can you keep that up? At 14 hours per grading period (five plus year program), you're okay. At 19 hours - it's really tough.
If I were picking again, I'd do my BSME again with some business courses, fly on the side, then pick up an MBA while instructing. Five year program (well, closer to six), two degrees, and my ratings. I actually did an MBA while instructing, but it was a Master of Beer Administration. Not a productive use of time.
All of these options take megabucks. Hope you have a scholarship available, or a rich uncle.
Again, the technical degree is for non-aviation use. Nobody hired me for a flying job because I had an engineering degree. It just checked off the "degree" box. It has been useful in other areas.
Like I said, college is a blast - but for it to be worthwhile you're going to have to work. Hard.