The most expensive orchid varieties in the United States are typically those with rare and unique characteristics. These include the following:

1. Paphiopedilum (Slipper Orchids): Known for their distinctive pouch-like blooms, Paphiopedilum orchids can command high prices due to their rarity and vibrant colors. The 'Bolero' and 'Dancing Lady' varieties are particularly sought after.

2. Cattleya (Cattley Orchids): These orchids are famous for their large, showy flowers and are often used in corsages. The 'Cattleya trianae' and 'Cattleya mossiae' are highly prized for their beauty and can fetch premium prices.

3. Dendrobium (Dendrobium Orchids): Dendrobiums are popular for their long stems and multiple blooms. The 'Dendrobium Phalaenopsis' hybrid is particularly expensive due to its rarity and the effort required to cultivate it.

4. Phalaenopsis (Moth Orchids): While many Phalaenopsis orchids are relatively affordable, certain rare varieties, such as the 'Phalaenopsis amabilis', can be quite expensive.

The high cost of these orchid varieties can be attributed to several factors. Firstly, the rarity of certain species and hybrids contributes significantly to their value. Orchids that are found in limited quantities or are difficult to propagate naturally tend to be more expensive. For instance, the Paphiopedilum orchid species 'Bolero' is known for its vibrant pink color and is quite rare, making it a high-end variety.

Secondly, the cultivation process plays a crucial role in determining the price. Orchids require specific growing conditions, including temperature, humidity, and light, which can be challenging to maintain. The time and expertise required to cultivate these conditions can drive up the cost of the orchids. For example, the 'Dendrobium Phalaenopsis' hybrid is a result of careful breeding and selection, which adds to its expense.

Additionally, the aesthetic appeal of these orchids contributes to their high value. The unique shapes, vibrant colors, and intricate patterns of these orchids make them highly desirable for collectors and enthusiasts. The rarity and beauty of these orchids often result in them being sold at premium prices.

According to a report by the American Orchid Society, the average price of a Paphiopedilum orchid can range from $50 to $200, depending on the variety and its condition. Cattleya orchids can range from $100 to $500, while Dendrobium orchids can vary from $30 to $150. The Phalaenopsis orchids, on the other hand, are generally more affordable, with prices ranging from $10 to $50.

In conclusion, the most expensive orchid varieties in the United States are those with rare characteristics, such as Paphiopedilum, Cattleya, Dendrobium, and certain Phalaenopsis orchids. The high cost is due to their rarity, the difficulty of cultivation, and their aesthetic appeal.