When do you start utilizing other paid SaaS tools in your product?

I have a very small project called Curlsbot. It's nothing fancy though it has zero costs (Github Pages) and makes about $300-$1000 a month. I am currently working to add new features and looking at Algolia. I know Algolia pretty well from my previous job and it would be perfect, but I'm worried about the cost being higher than expected. I could also try an OSS solution like Lunr and use it until I get a better sense of how many requests I'm getting, but that is also extra work.

At what point do I invest in such a SaaS tool like Algolia? If anyone has any tips or insights I'd highly appreciate it.