Something I'm sort of tired of seeing is that illustrators should be payed like plumbers. Why?
The problem is that illustrators think that they need to be paid. How much should you pay? The quick version is: It depends. Assuming a reasonably new artist, you should pay about $100 for a page (8.5"x11") of black & white art, or $250 for colored, regardless of medium. That assumes a piece of art that is finished; if it's just the rough art, where someone else needs to ink it, more like $50-$100. On the other hand, if you want the person to letter it, add $50; lettering may only take a few minutes, but there is some serious graphic design skills involved. So, assuming a full page of art, a person should be charged $50 to $300 minimum. If, on the other your project is bigger or smaller, pay appropriately; a full comic (assuming 20 pages) is easily in the neighborhood of $2000 to $5000, while a quarter-page (4"x5" or so) should be $25 to $60. For a seasoned artist, or even a famous one, that number really starts to escalate, so that keep that in mind.
However, that's the quick version. Here's my problem: An illustrator is not a plumber. A plumber does something specific that you need done, and without which life gets very complicated very quickly. Try not taking a shower for a week or realize that you cannot do something as simple as keep your house clean because of a lack of water. I need water; I don't need art. More to the point, if the plumber does not do his job, I can sue him and he is likely to get fired. However, an illustrator is hired in the hopes that he will do the job and make the comic come alive; an illustrator is hired on speculation of his doing a great job, whereas a plumber isn't. This should not be offensive to illustrators; art will always be based on how people like it and not how they do it as long as someone continues to like it. Seriously: If you saw a plumber pull out anything but a snake and a wrench and start working on your pipes with a large club, how long would that plumber be in your house? So stop with the plumbers and illustrators analogy; I get that illustrators need to get paid. But that's the only thing they have in common with plumbers.
The other problem is that writers and illustrators look at things differently. Screw the education factor; I see one more illustrator complain that he should be paid because he has a single Masters degree I will seriously debate putting out a hit on him. If you compared writers to illustrators, you would quickly find that writers have taken more classes and picked up more degrees than the average illustrator; most illustrators have pretty much just the art degree, whereas writers usually have a degree in something else, and then have to go back to school for the writing classes.
The problem is that writers are used to doing on spec. Illustrators are truly fortunate; they can find someone hiring, take their college work in a portfolio to the interview, and possibly get a job where they are paid for what they are told what to make; that is, you have a contract for work. "Spec" is where you put a year plus into something and hope someone pays you; let's just say there is a reason authors love print on demand. It gets worse when you realize that, in order to get some of the best writing jobs, you need to go through some pretty weird hoops. Try to become a decently paid screenwriter; in order to get paid even scale, you need to write AND get paid for a motion picture script (or equivalent, such as four half-hour shows) before you can join the Writer's Guild of America, and you have to do this in an industry where to have to be a WGA member in order to be able to even write a movie script. The same applies in a number of industries, such as comics. Getting paid is cool, but pretty much optional for most writers.
Now keep in mind that there is a part of us that think this "if you're going to make it in an industry, you need to be known outside it first" mindset applies to all artists. Ergo, we have no problem looking for illustrators stressing that this can be great exposure; if we're doing something for exposure, and that's how we get into the industry, why shouldn't you? Making the case for us is that this is how the comics industry actually works; you need to draw a decent comic before someone in the industry will take you seriously. There are exceptions, such as Kubert grads, but in general you need to show you can do a decent comic before someone takes you seriously. Now, if you happen to be one of the few decent writer/illustrators out there, this is not a problem, but if you're just an average illustrator you're going to team with a decent writer in order to create that decent book in order to be taken seriously; you can't just put something together and hope, because if the writing sucks, they'll likely toss the book quickly, and there goes your chance. The logic is pretty basic: omics are still dependent on the story, and great artwork only gets you so far. If you can't put together a decent book, which means a decent story, then the artwork, no matter how great DOES NOT MATTER.
So...where does that leave us? Oh. Right. Illustrators need to get paid. But...there can be times when you need the exposure in order to show that you can draw a decent book if you want to be a comic book artist, and a free job MAY do that for you. So, when you are presented with a writer looking for free art, debate it. Only if the story really grabs you should you go for it. IF (Note: Big if, not a little one) you decide to go for it, bargain hard for half the profit, and then go for it. But don't get into the habit; do it once or twice, and then stop. I mean, you do need to get paid, right? Right.