Every test I've done with Sonnet or Tera has been more expensive than just using their big brothers, even before 6.0 and 6.1 Sol came out at Tera prices.

They're cheaper per token, but they burn more tokens and take more wall time than just going up a tier at a lower reasoning level. And if the task is simple enough that they do come out cheaper than Sol, I can switch to Luna and come in even cheaper.