While I agree with jim in principle that the cost of doing business (CODB) is your floor for pricing, in some markets it is also your ceiling.  If you were willing to be employed by a company to do this job on a full-time basis for CODB amount of money (giving all rights and copyright to your employer) then why would someone be willing to pay more than that amount to own less than the whole ball of wax? And in general the likelihood is indeed low for reuse, unless you have a popular style or can turn to product marketing of the image.

It is very well to quote a price of say $3000 for a book's cover art, but if you are angling for a book cover on a 10K book run on a halfway obscure subject,  I suspect you have blown past the book's budget. Unless there is no way on Earth they can do the book without a custom piece of art that few people can generate.

Now if you are able to produce an accurate/dynamic color illustration of the Salmon in 3 days, and your COBD is $75/hour = $1800.

But if you COBD is $100/hr, and it will take you a 40hr week to do it = $4000, I suspect you are not going to get that job.  If you have lots of other opportunities to work at this rate, then you are fine.

If Nick is a fish geek and has the research in hand, and mad speed on his painting technique, he can do well with a $75/hr x 24hr schedule.  If like me, he is less than superman,  a $60/hr x 36hr schedule might be more realistic.

Then the question is, if the publisher is only willing to spend $1000, do you have a $75/hr job to spend your time on rather than the $30/hr job in front of you?

But at least having done the math, you are aware that you are not meeting your target for independence if you take it, and are blocking out other opportunities that might come a long for the time you are doing the $30/hr job.

Here are some other data points:
 FotoQuote offers a stock price range of $630 -$1260 for a stock photo cover on a 10K Hardcover retail book.  I think by default it plugs in 150% of this for assignment work. 

GAG book 11th ed.: 
Consumer Book Cover Assignment- $1,800 -$5000
Instructional Text Cover Assignment - $700- $1,500

Recent conversation - popular group of colorful birds, all species, illustrated book, inside page (I assume male and female in like like poses) -. $1,400 per species. That is $700/bird.

So what is the price that a piece of Custom Art can rise too before the publisher says "we will settle for less than the ideal art on our cover"?  I submit is is well south of 30¢ per book copy, and seems to hover around $1800 from the various sources I explored here.

If the price goes too high, the true floor is the licensing of the Archive Library print of Salmon laying eggs from Getty at $1050. Less than ideal, but on subject - bring in the graphic designer to make up the difference!

Britt
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Jim wrote:

I would argue that the price based on your hourly rate should always be the minimum that you charge, and should never be viewed as the “high end” of the price range. Think about it this way. You have calculated your hourly rate based on your actual expenses (your salary and overhead) and divided it by the number of hours available to work in a week (or a year, or whatever). As Britt said, this is your “cost of doing business.” If you charge less than what it costs you to stay in business - you won’t. You’ll be losing money and will eventually have to do something else for a living. According to a good friend of mine and successful freelancer, the main reason freelancers fail is that they don’t charge enough to cover their costs.