It's honestly differ depends on the style you're looking for. Cartoonist style? American comic style? Manga style? They're all different, and depend on the quality, the price differ too.
For example, I drew in manga style and for one full body flat color characters art, I price it at $55, some people can charge it lower or higher, and even will charge you the consultation fee because talking about the characters took a lot of time and a lot of reference exchange. If there's a lot of time when you want to change one characters art, they might also cost you some $ for that modifications (I let my client have 3 sketch modifications for free and more the more modifications, more money will have to get involved), so if you want to look for an artist, first check if the style you want is something they draw, then check the price and look at other with similar style arts to see your options and their portfolio and review if theres any (be careful to not get scammed!). Then if you know your budget and the artist you want is open for commission, try to read their term of service for yours and the artist safety (a commissions artist usually have them) before trying to offer your commissions. Having a contract for a big project is always a good idea.
For comic, you can always try to count it as 1 page with normal panel of 6, how many panel per page is one long scroll comic? They're different per creator so just made an estimation. If one panel cost like, $10 for that creator, you can start to count your budget. Once again stuff is different per creators so it's all depend on the quality you wanted and your budget.
High quality art? High budget.
Low quality art/just started out? You can get some very cheap artist to help you out.
This is probably kinda useless but I just rambling.