I don't have the last figures, but SiFive's annual revenue is roughly $120M from what I got, so that should give an idea.
$2B looks pretty over-valued, generally speaking, for a company with this kind of revenue, but still relatively common in this specific area, for a company that is probably still considered a "startup".
Certainly buying them just to avoid having to pay for IPs would be rather dumb. Especially since it wouldn't even get costs down to zero; you'd still have to pay for SiFive's operating costs (at least if you keep a significant part of it...)
Buying them to get the know-how, well that wouldn't be a good enough reason here either. I don't doubt SiFive's cores are good and that they have a bunch of great engineers, but I doubt it would be worth that much money.
So there is inevitably another good reason - as we suggested earlier.