So how much money can a celebrity make?

For me, it's making lots of videos and songs. It will take some time, but eventually the royalties become very nice.

You'll have to define "celebrity" first. Actors don't make a lot. You can only have one acting gig at a time, and it takes two days to finish each one. At best you will only earn three paychecks in a week, but more likely only two (with a pristine rep and one of the quirks I think you can skip auditions though?). The best I've had so far was in the ballpark of 9k per gig, as a 4 star actress. I've yet to achieve 5 stars, but the average is supposedly the same for 4 and 5 stars. That 5th star takes FOREVER!! ;)

Painting, on the other hand, can easily net you 10k per day as a 4 star painter and max painting skill, but painting and writing have always seemed like the biggest money making hobbies in Sims 3 and 4. Music with 4 stars was somewhat lucrative; I was netting around 8.5k per day from royalties on songs. I didn't see a whole lot of difference between writing/performing and the new music studio thing as far as money goes. I can't speak to the video hobby/careers, as they're not really my thing. Next I am going to try out writers to see what they can do with those stars. (I've heard they don't really benefit from them?)

Money Tree! > if you have seasons...