Well the way those things usually work is ICOS is entitled to royalty on the payments on the drug they developed and Lilly sells it retail, but you knew that I'm sure.
One of the reasons it's hard to value a buyout is that future cash flows are unknown, future demand is unknown, future competition is unknown, it all just goes into a financial model with various discount rates and time tables.
Sometimes buyouts are based on multiples of what competing firms are trading for, but in this case and in the case of many biotechs that is irrelevant, ICOS looks like it has a 100 P/E which just means it's an ultra high growth firm... no comps. really.
Carl Icahn tried to do this with one of my stocks last year, Fairmount Hotels. It was at $32 and he bit $40. He lost... after another firm saw what he saw they bid up to $45 and bought out the company... Icahn ended up making $180 million on his shares anyway...
Same thing happened to me with AX.
Long and the short of it is you're in a good situation, when two people are fighting over the price like this the odds are it will either stay where it is now or go higher... especially if this other company starts to acquire a larger stake and challenge Lilly... it's only a $2 billion company so that's not so far fetched.
If you are 4everhung, hold it, you've already got your shares and probably capital gains tax treatment if you've had it awhile. if you are a new investor reading this it's just gambling to buy it now.