I disagree with the best bit, but yes the most expensive is often true!
We aim for the best but struggle to build it. I've seen it all over - multi-role-do-anything specifications that are way over the top for the scenarios we actually have to deal with day-to-day. We then struggle to build things to those specs, thus draining all the money from the budgets.
The real cost is the manpower, their salaries and future pensions. By having multi-role equipment we also have multi-role people. That's kind of the unwritten requirement of all the big kit we build.
Same People, More Kit
Maybe the whole thing would be cheaper overall if we had single role equipment with multi-role people? E.g. pilots who can fly a pure fighter and also a pure ground attack aircraft. They'd have to learn both trades in a multi-role aircraft anyway.
It would mean a lot of kit being parked up for long periods of time, but that might be far cheaper than smaller quantities of multi-role kit that costs a ton of cash to make it work at all.