Look at similar tools and see how much they charge per signup. I would say maybe 5$ for something like this. Assume 5% of users pay which is industry standard. So that's 15$ per week or 60$ per month. Usually 24x of revenue is what people pay for tech products. So that's 1440$

The functionality is too basic to charge something for it. But I can imagine other ways of making money from it. I know the user acquisition costs for an extension like this can be between 1-3 dollars per user through paid mediums(my company does this). So 2k to $3k is what I was thinking as well