I've written about how much searchmysite.net has cost over the past 6 months, and how much it may cost to keep going in future:
https://blog.searchmysite.net/posts/searchmysite.net-the-delicate-matter-of-the-bill/
Short summary is that it is (perhaps) surprisingly expensive to run a search engine and search as a service.
The good news is that there is a plan to try to pay running costs (without resorting to advertising). To that end the paid listing option is now live.