The most expensive part of an excavator is typically the hydraulic system. The hydraulic system is crucial for the excavator's operation as it powers the various movements and functions, such as the digging arm, the bucket, and other attachments.

Hydraulic systems in excavators are complex and consist of components like hydraulic pumps, hydraulic cylinders, valves, hoses, and filters. These components are built to withstand heavy-duty tasks and high pressures, making them expensive to manufacture and maintain.

Additionally, the size and capacity of the excavator play a role in determining the cost of its hydraulic system. Larger excavators with higher horsepower and digging capabilities often have more robust hydraulic systems, contributing to the overall cost of the machine.

However, it's important to note that the overall cost of an excavator can vary depending on the brand, model, and specific features and attachments chosen. Regular maintenance and proper operation are crucial to extend the life of the hydraulic system and minimize long-term costs.