from what a friend told me regarding custom building your own tank, once you go over 19" in height, you will need bracing at the top. because the tank will be prone to bowing due to increased water pressure.
and i'm not sure if you would need to have just the bottom of the tank as tempered. of course going tempered all around does increase the price of the tank, and if you go with regular glass all around, the thickness is increased.
i myself am going to building a custom 50 gallon tank, but will be using regular glass, which will only have to be 1/4" thick. as for concerns about the tank bottom, i would ensure that you have a piece of foam insulation (1" thk) separating the glass tank and the plywood top of the stand.
is the $914.17 just for the one piece of 3/4" glass? that seems really expensive. i had gotten an estimate from a local glass place for my tank (36x19x19 - 1/4" thk), and they quoted me ~$150.00, which included a discount (~$180.00 before any discount). and that's in canadian dollars. in the end, the cost was cheaper than a store bought tank by about $20 dollars.
there has to be some local glass shops in your area that can cut glass for you for a better price. and kilroy's suggestion of GlassCages.com is not bad for prices in the size you're looking at.
anyways... good luck with your search and keep us updated