Does word count works with e-commerce and dynamic websites?

Yes, but it require more attention and more detailed previous examination about what pages need to be counted, what are the URL params need to be ignored, what content multiplicators (color, size) should be handled.

How long and how much does the word count take?

Each discovered page costs €0.001 / $0.0014, and takes, on average, one second to assess. Speed may be influenced by the target server’s settings and capabilities.