Engineering costs are capital/fixed costs, they're paid once to develop the technology and don't scale with the number of trips. Operating costs (which is what I'm discussing here) are what it actually costs to run each ride. Waymo's marginal cost per trip doesn't include a chunk of some engineer's salary.

Once there are enough trips, the fixed engineering costs are spread across more and more trips, exponentially trending towards zero, driving the cost per trip even lower.