I doesn't have to cost anything.... For ideas...
this members thread has a shot of his make-shift studio, its pretty much the same as all studios at its base.
.... you can use window light and a simple lamp for lighting instead of expensive lights, (move the studio around instead and use sheets of paper to diffuse the light softly) ... the main thing is getting that curved parabolic shape to remove any ugly seams
...some do this without the shape, just using glossy white for the bottom and back
or, in the other direction, absorbent black...this is trickier than the simple parabolic method I'd suggest.
You can use large coloured card or/and the plastic floor idea, or any mixture of anything clean and unblemished, your gonna need something waterproof for the water side of things obviously.
Hey, You could use a bath... errr, for the photography I mean.