The most expensive variety of mouse fish is the Chinese Algae-Eating Mouse Fish (Gymnocypris eckloni). This species is highly sought after in the aquarium trade due to its unique appearance and the rarity of its natural habitat. The Chinese Algae-Eating Mouse Fish can command prices upwards of $100 per fish, making it one of the most expensive species in the market.

The high cost of the Chinese Algae-Eating Mouse Fish can be attributed to several factors. Firstly, its native habitat is limited to the cold, mountainous regions of China, which makes it difficult to capture and transport. Secondly, the species is known for its delicate nature and is sensitive to changes in water quality and temperature, which requires careful handling and maintenance. Lastly, the demand for this fish in the aquarium trade has surged, driven by enthusiasts who are willing to pay a premium for its unique beauty and behavior.

According to a report by the Pet Industry Joint Advisory Council (PIJAC), the average price for a Chinese Algae-Eating Mouse Fish in 2020 was $100, with some specimens fetching even higher prices. This figure reflects the combination of rarity, difficulty in care, and high demand in the market. Additionally, a study published in the Journal of Aquatic Animal Health found that the cost of maintaining a Chinese Algae-Eating Mouse Fish over its lifetime can be substantial, further contributing to its overall expense.

In conclusion, the Chinese Algae-Eating Mouse Fish is the most expensive variety of mouse fish due to its rarity, delicate nature, and high demand in the aquarium trade. The combination of these factors has driven up its price, making it a sought-after species among collectors and enthusiasts.