I think the 20% is slightly too low, but at the same time 50% would be quite expensive as a deflected nuke costs 12 MAPs and is expensive to purchase. Perhaps 30% or 40% would work out fine
You also have not seen all the resources that Pol pot (over the last week) and ford (as of more recent) have been purchasing. That is arguably messing up the game more than just ford's nation. If you go banning some nations who abused the bug, damage was still done as that nation most likely purchased many resources from other nations, spreading their wealth
Adama is correct in saying that the cash has been dispersed in ways we haven't realized. Just look at Pol pots trade history, he has been messing with the market ALL week by purchasing resources at high prices. I think a roll back a few days would be the only way to fix this