Hell.....if you want to be cheap rent a bobcat from the ace hardware in navarre, and go online and buy a 1000 gallon Poly septic tank for a thousand bucks.
http://www.plastic-mart.com/product/5756/1000-gallon-septic-tank-1-compartment-41732
Dig a trench 6 feet wide or trench and go to the water level of your soil, or deep enough to cover the septic tank. Get a saw and cut a three foot by three foot hole on one of the two hatches on the tank. Get 8 4x4x8' treated lumber posts and set four corner posts. and keep them plumb and backfill your hole with the bobcat. Compact around the sides of the excavated area keeping the four posts plumb. At grade set your remaining posts to create a box which is bolted into the four posts and box around your 3x3 open cut entry. Get spray foam after carefully placing plastic around any imperfect cuts. Order a yard of concrete. Make sure that you also purchase a two inch rubber boot to fit in the two inch vent pipe location, and buy four feet of 2 inch pvc with a 90 degree and put a standard dryer bug screen... set your pvc and pour four inches of concrete in the framed area above the tank and set four stainless threaded into the concrete around the three by three. A half yard of concrete should be plenty, but if you wanted to make this really secure, purchase four 8 inch sonnet tubes at lowes and place them in first, and then set your four corner posts.....then pour the balance of your concrete in the sonnet tubes......this thing is going nowhere, and all you do is purchase a three by three steel hatch. You may want to go with a threaded steel vent pipe with a 90 degree for a few more bucks, and this would not break off during a tornado, and if you set it high enough.....even a storm surge is not going to get water in this tank. I am thinking about building one of these, but I have a basement in the house I am living temporarily in now.