Vector Stores are charged per gigabyte per hour that they persist. So 8,000 small vector stores will only be expensive if you make them persist forever. I think they give us 100 free GB of persistent File Storage. (But the cost should be the same even if you have one large VS with 8000 files.)
But, TBH, mate, I think your needs are already exceeding capacity of the platform you’re trying to use. Input Token / Output Token counts are a standard statistic that the API response includes—so you’re just waiting on the producer of your platform to include the statistic.
I suggest doing your experimentation on the platform.openai.com. I am far more comfortable with Graphical User Interfaces myself. Even now that I’m learning to code Python, I still use the GUI. (I usually have vs code on one screen and the platform on another.) They’re doing a good job keeping it on par with coding access, too.