I've made 2 of them for friends for about $10 worth of spare parts that I didn't have already in the pile. It' might be worth around $100 if you need to buy the parts from a junkyard and buy welding services from someone though.
The first one was just a 5 gallon plastic bucket that I filled with concrete and put a 2" diameter galvanized pipe into. After that set up, I filled the pipe with concrete (could use sand or lead shot, etc. too) as well. The top end of the pipe was threaded and I just used some fittings to reduce down to the size I needed. Then welded together some 1" steel rod to make some toolrests. Quick, easy, cheap and heavy enough to not want to move it around if you don't have to.
For the second one, I substituted an old milk can (it stands about 3' high and is around 18" wide) for the plastic bucket and filled it up with concrete. I also used some 4" diameter well casing for the stand inside of that (again, filled with concrete). No threading on that so I had to reduce to the size I needed for the toolrest insert with some steel fittings welded onto it.
They're both round at the bottom so if you had (HAD!) to roll them out of the way a good distance, you could. But you really don't want to if you can help it. On a good floor, there's very very little vibration felt either.
For further support, you could always setup a swingarm and brace from these to the lathe legs. It'd be more tied-in to the lathe that way but you'd want them low enough to not interfere with the spinning piece.
Just an option.