I'd definetly do some more shopping around. You may have to make a road trip but even in a big ol corn field called Iowa I can get a reputable big tank for significantly cheaper.
I recently aquired a 300 gallon Tenecor with twin trapezoidal overflows in very very good condition with a stand for barely over 1/2 what you are talking for a DIY tank.....
I've read that you should not use extruded on larger tanks. I would be very hesitant to build a tank that large without having any experience, but that's just me. I guess the risk depends on where you're putting the tank. Do some searching on reefcentral.com. They have a few threads that are dedicated to DIYers building acrylic tanks. There's a ton of good info there. My advice would be to practice with a few smaller projects before building such a huge monster.
well if i do end up buliding it im sure ill be seeking a lot of help from others and businesses around the area. and as for cali. its a ways.... for me i can get to yuma az in about 5 hours going OVER the mountains... so id say roughly 7 hours goin south... northern cali and la area prolly 8 hours...
If you are going to build a big tank. Start with 1/4" sheets and build a sump for it first. That way you get practice before you attempt the bigger heavier and more expensive sheets.