Per successful tasks. The result are mixed. Like you mentioned, it can be cheaper but only in some use cases. I'm only on the $20 plan. If I use Opus and it's not as efficient for my current tasks, I'll burn through my limit pretty fast. Ended up can't use any anymore for the next few hours.

Whereas with Sonnet/Haiku, I'm much more guaranteed to have 100% AI assistance throughout my coding session. This matters more to me right now. Just a tradeoff I'm willing to make.