Actually its tough to say b/c we only know 1 of 2 things:
1) The supply will extremely low by the year 2034. Barring the Gov't forcing cars that use petroleum based fuels off the road or something or to be crushed etc - who knows, cash for clunkers was a good start.. ridiculous
2) And that leads us to not knowing what the demand, if any at all, will be for them.
so roughly adjusting for Inflation, yep I could see them going for $20,000+ assuming, like you guys mentioned, if they are legal