I'd guess somewhere between $5m-$10m.
Given the number of employees and the amounts they've raised I guess their burn rate was around 250-300k/month (that's probably on the large side). Their series B was only raised 6 months ago so based on their burn rate I'm assuming they've still got 3m-4m of that in cash so that would be the lower bound for the acquisition.
Given the company probably had 12 months of cash left I suspect the Series B investors would only have agreed to a sale if they got all their money back (they'd have liquidation preference anyway so they'd get their money back first), so that probably means a lower bound greater than $5m (the size of the Series B).
Anything over $10m would be high for a talent acquisition so that's probably a decent upper-bound.
Liquidation prefs are rarely exercised, at least when the acquirer is clueful. (The company spending the money wants the incoming employees to be reasonably incentivized, often that takes the form some delayed payout. They really don't care if the investors make their money back or not.)
It's usually among the first things to go in a negotiation.