I suspect the true total cost was a fair bit higher due to a hosting arrangement. I'd imagine Tumblr remained on Yahoo bare metal servers for quite some time after the sale. If so, Automattic would have had to pay a significant monthly fee for this, given Tumblr's large infrastructure footprint.

That arrangement would have been quite appealing for Verizon, since by this time there was likely a surplus of aging bare metal servers in Yahoo's datacenters, and these would otherwise be difficult for Verizon to monetize.

disclosure: worked for Tumblr but left long before this sale and not directly familiar with any of the specifics of the deal