Quote - 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%.

 

I second this notion.