I have been doing commissioned pieces and occasionally selling original works and prints for about 7 years now.
What I found is what Tiny has described above - start lower with your prices, build your name up and then start to increase the price.
I go by 'supply and demand' - if you have a huge waiting list then you need to increase your prices. If no one is interested then you need to lower your prices until you get some interest and start getting some word of mouth advertising.
Initially I had to do a lot of advertising, I paid for magazine adverts, got a lot of business cards and flyers made up, etc. It took a good few years before I built up a steady stream of work.
Now, I barely advertise other than occasionally sponsoring an event of donating a piece of work for an auction for charity. Everything else is word of mouth. I have just had 8 months off art to study, have just started up again and my waiting list is now at 12 with 5+ enquiries sitting in my inbox. It keeps me very busy, and as of Feb 1 my prices will go up again as I cannot keep up with the demand at present.
Regarding prints, they are good if you have interest from others in a portrait. I have had some made - you need to do them professionally, taking a photo and getting it printed doesn't cut it. I go to my local photograph printers, and he has a camera designed especially to take photos of artwork for prints, it is essentially a scanner without having to touch the portrait itself.
This process is reasonably expensive, then you have to factor the cost of the prints as well which are not cheap, depending on what size you are doing.
When doing prints, you can't do them of other people's horses unless you have permission to sell them, and the golden rule, NEVER advertise or print/sell any work that you have based from a reference photo of which you do not have the photographer's permission, in writing. This is very important!
Selling prints/original works (non-commission) is very difficult, people have to like the work enough to hang it in their house, and when its not their own animal or a very famous animal which they have an attachment to, then they are not interested unless you work is of amazing brilliance and very original.
The only prints which I have had success in marketing and selling was a portrait I did of champion Australian racehorse, Black Caviar. I could have sold the original 10 times over, and went crazy with selling prints. Pick your subject ;)