Unless an e-commerce company is transparent in sharing their sales details (this is probably very rare or unusual), you won't know the metrics needed in order to calculate it.

I would think that the 2nd best thing is to hire a scraper or data entry person to collect the average item prices to try to build an idea for it.

Sorry, that is the best idea or help that I can offer. Perhaps this is an opportunity for an entrepreneur to build this. :)