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I think the best solution for a cheap mirror is spinning liquid mercury. As a largish mirror, its 1% the cost of a glass one. The mercury only needs to be a mm or so thickness on top of a rough form work. Mercury is about $200 per kg. I dont know, perhaps you need 2 kg for a 20" mirror?.
It needs a motor rotating a few revs per min to maintain a parabolic shape, thats not hard, it seems to be a no brainer really.
A very small, trivial problem is it can only point to the zenith, but you can pan in post with a large chip cam, so thats covered. The kind of monster apature you can afford with this kind of mirror would make exposures pretty short anyway.
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