Samurai said:
First, no artist draws in 2"x2" size... all of those monster pictures are shrunken from full page size. If it were actually drawn at only 2"x2", the picture would have little or no detail and probably look terrible.
That said, a monster pic typically has little or no background, which makes it easier to draw. You might get a discount for commissioning several pictures at once, too. I have charged about $30-50 each for the ones I've done, depending upon various things like color/b&w, amount of background, etc . That is about half the industry standard, but I'm just starting out, can't really charge full price yet 
. Getting there though...
Well, that first bit might be accurate as to how an artist draws or scales the art but there is a difference in their personal taste of "how to" and what they deliver to the publisher.
We pay in increments of 1/4 page, 1/2 page, and full page for interiors regardless of how the artists decided to actually draw the picture.
The art prices vary greatly. $30 dollars is acutally getting high these days with the average being $25 for a 1/4 page B&W picture. In bulk we go as low as $20 or $15 for a 1/4 page. What I mean by that is we often commit large chuncks of work to one artist and offer a bulk price for the workload. Sometimes even multiple projects. This is a win/ win for everyone.
The only interior artists we will pay more than this for are those that have a big name. People who actually help sell your titles just by being associated with it.
On covers, the prices vary so greatly I can't begin to tell you. We will obviously pay more to a cover artist who has a big enough name to actually draw sales (examples would be Todd Lockwood, Jason Engle, Brom, Larry Elmore, etc), While undiscovered or upcoming talent gets considerably less than those with an industry name. That is not to say they are better or worse artists, just not as popular by name.
There is also a matter of rights. We may adjust the pay scale depending on the terms of the rights for the art. In other words, who owns it, who has a copyright on it, and what can the original artist do with it. I won't go into detail here but generally we seek first rights. We won't pay nearly as much for second rights or recycled art.
We look for full ownership/ rights on logo art and title treatments, etc. so we pay a little more for this in some cases but the artist loses all rights (other than being able to display it in his or her portfolio).
Hope that helps some.
