I read a recent thread over at HSM, where another poster was thinking of doing the same thing. Though he wanted the X/Y table to mount to his knee mill, I thought it was one way of having CNC ability, that could be easy to add on, and remove at will.
On that topic, the quality of the X/Y tables were talked about. There is many out on the market, in many sizes. I would imagine a decent quality table would be a big difference here. You didn't mention if the table you were looking at had ball screws installed or not? I guess if these tables come equipped with the ball screws, and steppers installed, that would explain why the price is up there.
I thought it was a neat concept for a full size knee mill, but your thinking of adding this to a 3 in1 , so you may have other things to concider as well.