What is the most expensive part of Landscaping?

You could break it down into categories.

Earthworks

Hiring earth movers to do the ground works is a significant cost. Not only do they shape and move materials,  they also remove the spoil which incurs a tip fee.

Materials

Materials can range in quality, availability and cost. On top of that they can fluctuate in price especially when you face supply chain issues and high fuel prices. Materials can make up more than half the cost.

Variations

When changes are made half way through a job this has a knock on effect that can affect the finished timeline.