as said here:
it is worth how much one would give for it, like everything.
Exactly what it's worth. To figure that out in hard numbers given the vastly different needs of someone who doesn't need to eat, sleep, or use the bathroom, and someone who does need all of these things, is like looking at a picture of a square with no background and asking "How big is that box in real life" without any artist to tell you the answer. A toilet in Skyrim would likely cost nothing, as it's useless. In reality, it's almost invaluable as a part of today's society, though supply and competition bring it down to a reasonable price (Or regulation/supplier's best judgement if there is no competition).