Clone without the cost.
The Workflow
When duplicating an object (like a rock or tree), use Alt + D instead of Shift + D.
The Tech
Shift + D creates a new object with unique geometry data.
Alt + D creates an Instance that points to the original data.
You can have 10,000 Linked rocks in your scene, and Blender treats it as 1 rock in memory. If you edit the shape of the original rock, all 10,000 update instantly.