It's all just speculation, but here's a wild swing at a stat ...
Estimate 3,250,000 "assault weapons" in the US (and that's a low estimate). Estimate an average cost of $1000 (or is that too high?). That would mean it would cost the government approximately $3.25 billion to "buy them back."
I don't see that happening. I think either grandfathering or an uncompensated ban are more likely scenarios.