I'd thought they'd talk about the whole value of the company instead of the price per share.
In other words J&J says $6 Billion Momenta says, nah we want $7 Billion (really should have been at least $10 Billion). Then they mutually arrive halfway at $6.5 Billion. And it so happen it works out to $52.50 a share.
It works like that in real estate deals. I thought it would be the same.