I got the glass from a salvage place, so it is used glass (recycling!) and I did not have the place polish the edges for me, as that would have been an extra $14, I did it myself with a piece of 360 grit wet/dry sandpaper on a block with a little water, it took down the edges nicely.
So the glass was $16.50 and the silicone was $5. Next time I am just going to buy a sheet of glass and cut it myself though, as I was kind of unimpressed with the cutting job they did. Some of the corners weren't square and the edges weren't flat and some of the pieces weren't quite the right measurement. None of it was off enough to effect the finished result, but if I'm going to do more tanks with smaller, cleaner seams, I'll have to have perfectly square glass that is cut at exactly the right size.
I got the idea to build it from necessity, really. My mom has orchids in her window, but can't find an attractive humidity tray, so I decided to build an aquarium for the orchids to sit on.
As for how, I read the sticky thread in the DIY forum here for the 75 gallon custom rimless and got enough info from there to figure the rest out myself.
Thanks for the comments everybody, I can't wait to get it scaped! But I have to go clean and prep the wood first and finish leak testing for another 6 days.