This is of course not including the yearly Unity subscription, where Unity Pro costs $2,040 per seat (although they may have Enterprise pricing)
Absolutely ridiculous. Many Unity devs are saying they're switching engines on social media.
My guess is Source 2 just not being ready until recently. They originally started work with UE4 but made some of their work engine agnostic so they could move to Source 2 when they were able to.