I don't think the market can properly gauge the value of IDCC's patents to the winner over the next 10-15 years. Google won't let Apple get IDCC for cheap.
I don't agree with the 80-90% rule you're suggesting. Before the news of a buyout, MMI went for 63% over it's last trading price, and Nortel was expected to fetch around 1.5 B before commanding 4.5 Billion. So what the hell does the market know if it's all secret behind closed doors?