I came across Waterfox a number of times over the years, but I think it will be difficult to get a similar amount of reach for your search engine. In particular, on the home page of Waterfox, there is nothing even hinting at the existence of the search service. Maybe this is intentional, as it is in public beta for now, but I think it would help to at least note its existence there, or near the `donate` section (as a means of support, rather than direct donation). Also make sure that this directly exists as one of the search engine options for Waterfox, if it isn't already, every click involved in the setup will make it easier for people to try out.

But charging $5 / $10 for basically what StartPage does (to the best of my understanding) is going to be a tough pitch either way. Out of interest, what would the pricing for the Google API look like, if you had no other costs involved?