Rough figures:

As a plasma donor you can earn $30-$70 per session for 800 ml. Let's call it $50. A session takes about 90 minutes, or 533 ml/hour, and you make $33/hour

Waymo charges $0.50 - $1.00 per mile. Let's use the high end.

To break even, your Waymo will need to consume < $33/hour, or < 33 mph. That's not bad!

If you go any faster, you won't be able to extract enough plasma in the same amount of time.