Self-balancing robots hardware - experienced opinions wanted

Could someone with real experience with self-balancing robots give me some idea what reasonably priced hardware - the body plus the motive power - is available in kit form?
I am looking for "table top" size and just the basic frame, do not need any sensors etc.
So far all I am finding are "ready to run " toys and I don't care to spend my $ on some flimsy gadget.
Thanks for reading.

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I built my first using the Lego NXT. It is a great platform for learning and experimenting.