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.