I didn't say they were unpredictable, I said they were uncontrollable. Whatever the cost the team has no choice - they HAVE to have the engine. Other costs are more discretionary.
I get what your saying, it isn't a cost they control, but it's not like it's unique and everything else is optional. The team
has to have 2 drivers, they
have to build a chassis to meet the regulations, they
have to pay for tyres.
They also have choice. Unlike Pirelli, if a team isn't happy with their engine supplier they can move. That's made even easier now with the standard mountings.
So yeah, while they are not in control of the cost of the engines, it's not unique in all the things that a team has to pay for, its not unstable like some of the others, and they do have some degree of choice which is something they don't have with other costs.
The cost of the engines will not have been a surprise to any teams, and factoring it into their future planning is fairly straightforward, relatively speaking.
What they won't be able to predict, however, is the complete unknown of a kneejerk reaction to a different engine format, with no idea on that cost, or how much if the lost investment the manufacturers will want to recoup through inflated prices. If Mercedes are expecting to earn $20m per customer per year for the V6, what's to say they won't charge $25m if they go back to the V8s? $5m for the "cheaper" engine, plus the $20m they need to cover the loss of developing an engine they then can't sell. They aren't going to just write that investment cost off. Mercedes, Renault, Ferrari and Honda have developed these engines under the expectation that they will be selling them to customer teams up too and beyond 2019.