Length & Size - Longer animations and higher resolutions are more expensive in general. HD/ Full HD/ 4k/ 8k influence render time.
Shorter projects and standard resolution are less expensive.
Content & Details - More details take more time to process.
Complex objects, detailed environment, more realistic lights.
Sometimes less is more, and more efficient.
Effects & 3D Motion Graphics - are very time consuming.
Effects need more power resources, especially if the scene becomes more complex.
This makes the price higher.