You are very unlikely to find a "cheap" tablet to run unity, this is due to graphics I would think.
While modern tabs are quad core the processor lacks high frequency, to clarify my PC has a clock speed of 3.0gz per core but even an expensive tab will likely only have 1gz this means they can multi task but are not great a intensive prossesing and the graphics chips are just rubbish, my pc has 512 dedicated mb for graphics, laptops will be less (look up intel integrated graphics) and tabs would be lucky to have half what thew laptop has.
The result of this is that the CPU must help the GPU slowing down graphic rendering and taking resources from the OS.
I think you could easily get Ubuntu on to one but not with Unity or KDE, you would need X or L Ubuntu.
Alternatively check out Fuduntu it uses the old Gnome 2 desktop which is fast and customizable but designed for less powerful (older) PC.
Another option is Bhodi linux with the Enlightenment desktop (E17) which has a special set up for touch devices and is still Ubuntu underneath.
EDIT : You will be able to get one next year though Ubuntu 13.04 will (apparently) get a mobile version read THIS and THIS