How much revenue should the average $50k website/online business make?
The cost of a website means nothing. What matters is how you deal with the site. My wife bought a domain name, spent a few bucks on a WordPress course, then a few dollars for domain hosting. We calculated that he cost is about $12. per month. Based on the particular niche that she fills, she makes about $1000-1500 per month.
A website's revenue isn't decided by the cost. It depends on what you are doing with that website. If its an online shopping website or a blog. Website's cost depends on how much storage it needs and how much complex features it needs. For example, a simple company's website will be cheap to build and an online shopping site will be more expensive since it has a lot of technical features.