I don't know exactly but I imagine the artists get a compensation package composed of an advance and % of sales. And also non-exclusivety so they can work on other titles.
It's possible to self-publish at the quality of Euro bd in the States. I've seen hardcover "sketchbooks" by Ryan Benjamin for example so if we're talking about a similar quality it's within reach (news of affordable pod lately too). But in a corp setting like Marvel and DC, I think the current practice of monthlies then trades make more financial sense than investing more in big projects similar to bd production and waiting over a year to find out if it made a profit or not.
The money is in the licensing anyway, some people say.
This is the Barbara Canepa that does SKYDOLL right? Peep her blog or better yet ask her!