I agree, it doesn't cost 2x. An SPA probably costs more like 3-4x.
Coordinating two apps is much harder than one app.
A reference for this topic is the Mythical Man Month, which concludes that communication overhead is what is expensive. When you have two apps, the humans, and the software both need to communicate and coordinate.
When there's two apps, you have an OR condition where if either app is broken or in disagreement about the expected API, both are broken. You have two sets of build processes, sets of tests, sets of files to serve, caches to manage, state to manage, versions to manage, and they all need to coordinate.
Instead of one thing to test, with an SPA, there's really 3! The server side app in isolation, the client side app in isolation, and the combination of the two.
Another locus of optimization might be development speed and cost. An SPA is really two apps, a front-end, and a back end.
It costs 2x to do an SPA, and all things being equal, features come out 1/2 as fast.
Which wins the app with latency, or the app that gets features twice as fast?