The plywood itself would probably run you about $45 for one and a half sheets. Maybe another $5 for 2X4s to reinforce the corners, and $7 for screws and glue. Gravel, rocks, and driftwood could run from free to quite a bit, depending where you get 'em; let's say $20.
That's $77 so far. You can probably add the basking light too before you break $100.
I don't know how much for epoxy; maybe Sploke can tell you that. And then you have a wide range of filtration options. Still, I think you can have the whole thing built and furnished for under $250. Not too shabby!
That size container would be fine for a single adult. I think it would be a little cramped for two. If you could upgrade to 4' X 4' X 2', or 3.5' X 5' X 2', that would be a big improvement. A huge increase in water volume, and essentially the same cost in materials.