I used this method very successfully for a number of years. Select cultivars that you have seen growing well in another local garden. These make a good choice because you already know that they perform well in your area. Finally, get recommendations from daylily growers in your area. Daylily folks are a friendly lot and most will be more than happy to tell you about their favorites.
Daylily prices can vary from a couple of dollars up to a couple of hundred dollars for a single fan.
Basically, it's the law of supply and demand. It can sell for this because it is rare. As more growers obtain this plant and multiply it, it becomes less rare and the price drops. If the cultivar is very popular or if it multiplies slowly, the price stays up longer. Likewise, if the daylily multiplies very fast or is not that poplular, the price will drop fairly quickly.