Most professional artist will at some point be commissioned to create an artwork, this is the bread and butter of earning money and just like any work has ups and downs. I have had done many commissions and some are just better than others. Knowing what type of patron you have is vital to how you deal with the job, and indeed if you should take it on at all.
The best commissions come from patrons who know your style, and really want something unique. They trust your judgment and leave the overall detail and design up to you. It gives you freedom to explore the subject in your own artistic way, and create something you are proud of and therefore something the customer will also treasure. These commissions area breeze and the price you charge should reflect this, you don't want to under charge, but you'll defiantly want to have this customer back for more, so give them a good price.
Difficult commissions come from patrons who give far too much direction, specify all the details, write long briefs that give no room for creativity. These patrons, invariably will complain when it's not exactly what they had in their own imagination. The atmosphere of this constraint tends to stilt the final piece. while it may end up being acceptable after much fraught teeth gnashing, soul searching and reworking, the final piece for me is always tainted a little. When negotiating you commission you will notice the amount of "things they want included and the amount of detail and specifics they ask for. Every time you hear a new one, you should be adding to the price you're going to charge them.
Finally there is the worse commission, where the patron ignores your style and asks you to paint, like someone else..? this is to be avoided at all costs... it doesn't advance your style, it's unlikely you'll create anything you are proud of. It's just insulting and the final pieces will be horrible. The best you can possibly do at this point is to find another artist who does the style the patron is asking for and see if you can get a small finders fee from the prospective artist for the commission recommendation. The worst thing you can do, is actually do the commission yourself. You won't be happy with the final result, the client wont be and anyone who looks at it will think you're a terrible artist.