$200 for 20 illustrations is a ridiculously low price - so low you're basically doing it for free. I would never volunteer for that job at that price...you've got to think about how long that would take you, taking into consideration that you've got a (demanding?) client who will need roughs, then amends afterwards - all that needs to be factored into your 'hourly rate' that you'll get afterwards...if you factor all of that in, x 20...your $200 will be nearly nothing as an hourly rate.
I just wouldn't touch it. Anyone who's asking for such a low price is being utterly cheeky and is going to be more trouble than they're worth (even if they up their price). You'd be better off spending that same amount time doing your own projects, that showcase your work better, and could then lead to landing a better, more professional client later.
I actually don't have so much practice at quoting for projects, since I work for myself...but I'd say don't sell yourself short. There are many ways of making money out of art, you don't have to go for the lowest bidding price just to 'get practice' and to feel like you're professional just because someone's paying you (a cheap price like that is not proper paying).
Sorry any negative vibes from this post, they're not meant to be directed at you - of course if you really want to do a project, and it appeals to you, and you have the time, then you should...but some people are just taking the 'p' with their offers.