Given the rate of discoveries, there ought to be new species out there not yet in cultivation in the western botanical world. There are still many places that haven't been explored, either due to lack of accessibility, or are just so dangerous no one in their right mind would go their (like some parts of PNG and The Philippines).
But there are at least two species I know of that are not commercially available, that are only grown by a handful collectors, that everyone thinks are still out there waiting to be rediscovered. Remember, it's not only nursery people who wander through exotic and dangerous places looking for plants.
As for campanulata, it would be the same thing that affect prices. Availability and competition. If only one supplier has a species or variant in cultivation, then you can almost guarantee it will be expensive. As soon as there's more than one, competition can start to bring prices down. Tissue culture certainly plays a large role in this process.
As noted elsewhere, it is not cheap to find and breed new species and cultivars, and it is sometimes very dangerous, and nurseries need to recoup costs and make money. And it's part of a market economy for prices to be a high as demand will allow. Given that there are a fair number of collectors who will pay big prices, prices will often be big until their demand is sated.