Why not call around to professional graphic design companies.

Don't tell them you are new and have been offered work but don't know how much to charge. You won't get any help that way because them helping you is the loss of a potential client.

Instead ask them from the perspective of someone needing a quote for a project you want done.   IE: comic book 129 images ....basically what you have been asked to give a quote on.

Once you have a price, undercut it by 10%.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi