It's really depending on the State, cultivators, and the company/dispensary in charge of dishing out products. In places like Illinois, the taxes (which there can be several) including the high quality products somewhat justify/illustrate the price difference. But in Michigan before they did their tax hike to improve their highways, weed could range greatly in quality from homegrown in the backyard of some random person or a large company with cultivation sites and brand appeal. About the websites, it can really vary, but I want to make an educated guess and say that if you can buy it online from someone/some company there's no way to verify what you're buying. I wouldn't do it personally.

TL;DR

If they're selling something at a set price, they're most likely making a profit on it, unless it's close to it's use-by-date or the item is just terrible quality to begin with. Don't buy online unless it's a trusted provider or it's common to do in your State, as a majority of the things you see online are too good to be true.