Mike raised a great point in one of the comments, and I want to bring it up here. (There may be a slicker way to do this--but I'm new to this.)
Mike noted that:
"...I don't want to be a freelance or hired illustrator for, as you pointed out, I wouldn't get satisfaction from a 'job well done,' no matter how well paid if I was forced to draw things other folks want. It's a drag, and the few times I've been hired that way I've hated every minute of it."
I think part of we are seeing is that external pay can represent external obligation, and that's no fun--it means it is not yours anymore, you've been disempowered. And considering that a critical motive for visual AND verbal artists who display their work is the Power/Influence motive, that means disempowerment is loss of motivation!
Mike continues: "But I still want to get paid for my art. It's a form of recognition and while monetary, represents an emotional fulfillment as well."
Excellent point! There are clearly people who get paid who do enjoy it, and comments on this blog notwithstanding, there people who have fun with their jobs and still get paid! Why should art be different from any other vocation or even job? I love my job, and I still get paid for it. I think this deserves more study, but I do have a hypothesis or two. You should feel free to propose more or different ones!
1. Some people can "detach" the joy of the process from the monetary result. I know a few professional writers who love writing and also enjoy getting paid, but they keep the two separate. They enjoy the process as it occurs. This works better if (a) you have the ability to compartmentalize (some don't), and (b) you are productive enough not to worry about deadlines too much. For some people, deadlines are anxiety-provoking and inhibitory (see Yerkes-Dodson Law), for others they are motivating, as in your pre-con productivity described below (also see Yerkes-Dodson Law!).
Every year, I get ready to display artwork at the con. Usually, there's a little rush of productivity right before the con and I get more done then than any other time of the year. I also usually do my best work at that time. While I do work to make me happy, I also hope it gathers attention and that I can sell some of it as well. This is a somewhat different situation than just having art as a job...But I can't help but feel a connection in the motivation between me wanting to do good work, be recognized for what I want to do and to get paid for it. Perhaps I'm missing a point here, or perhaps the visual arts differ in some manner from writing?"
2. My second hypothesis: people can also use the external reward as an intrinsic reward if you perceive it the right way--so instead of seeing it as an external obligation, you see it as feedback on performance. Salespeople on commission often see the money not just as money, but as evidence that they are doing better than the goal (Achievement motive in action).
So you can either detach the two elements, or link them selectively. In either case you need to manage your perception of the money, or the contract, or the requirement (if any). It sounds to me that in some situations (e.g., cons), you can do the work regardless of the money because you know the work will be seen, but once that is established, you are comfortable with thinking "gee, I'd like to get paid for that!" Pay can indicate that people value your art enough to pay for it! That makes money into feedback around excellence rather than a demotivator. Especially since, if I read you correctly, you are not obligated to anyone pre-con, you have just done your work and people can take it or leave it.
And in case I haven't made it clear, no, I don't think the visual arts are that different from writing--or from the performing arts, for that matter, having done a good bit of that.