Given how easy it is/will be to set up your own AI SaaS chatbot over time, I expect cost to consumer to trend very close to cost to provide the service. Many companies already have offerings with only a few to few dozen employees.
A few hours a year of compute seems a realistic median usage, so you can do the math. But it's not much. Certainly a premium AI that takes longer to process will cost commensurately more. But again, likely 80-20 rule applies. These more advanced AI aren't likely to be orders of magnitudes better than the cheap/common ones.
It will likely also be common to tier usage such that you only employ more expensive/computationally intensive models when needed. GPT-3.5 can already adequately cover a large number of use cases.
