A 90 gallon would also be an upgrade from 75. Exact same size footprint, but taller. You could probably also reuse the equipment from your 75 on the 90...same stand, hood, possibly heater. Might just need to add a little extra filtration.
I'd guess that in Dallas, you could easily pick up a bare 90-gallon tank for $150 if you watch craigslist. When I was in Chicago, I picked up a complete 90-gallon setup with stand, sump, and lighting for $250 total. Here in Alaska, prices are higher and there are fewer large tanks to be found. Recently got my 75-gallon with stand for $150, but I haven't seen a 90-gallon advertised for less than $500!
Just check every day until you find a good deal. Used is definitely the way to go...tanks are so big and bulky that people rarely take them along when they move, so they are always being bought and sold for well under the price of new.
You can probably also plan to sell your current tank once you get your new one, so that will offset the cost some.