It really depends on what you want to sell and how. It's a lot easier to resell then grow plants from seed yourself. For instance, Sarrs take about 3 years to be large enough for most people to want to buy. That's three years of space you have to dedicate to plants that you'll sell. Considering most Sarrs will sell from $15 - $25, that means you're making $5 - $7 a year per plant from seed. It's a heck of a lot cheaper to import some cheaper plants from elsewhere and resell shortly thereafter. If you can import a Sarr for $7, selling it for $15 a couple months later is a big profit compared to doing it yourself (but it's still not a big profit).
What sets the price of the plant is what people are willing to pay. I sell extras from my collection, mainly so that I can afford to buy more plants/plant supplies/etc. When I set prices, I take a survey of US and European nurseries to figure out what any given plant is selling for and weigh that against the time it takes to propagate a plant. U. livida, for instance, was spreading like a weed for me a while ago. Therefore, I could price it very cheaply. In contrast, U. fulva is rarer and is slower to grow. Therefore, I price it more expensively. Of course, what happens with plants, especially perennial plants, like most CPs, is that if you do your job well (and your buyer cares enough to do some research) you'll never sell the same plant to the same person. Since the market is small, you'll always have to come up with new plants to sell, or wait for a new crop of growers enter the hobby.