Depends on what you're doing. For example, I'm having one chat extend/optimize a (mildly-novel) CUDA program. Opus 5.5 (high/xhigh) is very efficient, but a single thread would still cost me the Pro plan usage in ~2 days (less if I were just running it unattended). This is one mostly-bounded program, and it's not even that large. In fact, before 5.5, I would have just used Fable instead.