It's been my experience that the prices initially set for the plants are estimates of their value, and the price appears to be based upon such things as the number of flowers that the plant will have when it's mature, foliage configuration, etc. You can almost always sell a plant at a higher price than the one initially set for it, especially if you have lots of people coming to your nursery.

When I'm breeding for money, I self-pollinate only the highest-priced plants. They still sometimes produce 'inferior' seeds that aren't as valuable as the parent, but (in general) the seeds from that effort will continue to produce more valuable plants (to a point) - you're never going to see a high price tag on a common plant.