What is your stack ?
I made a simple Proof-of-concept of a PHP backend based on Redisearch compatible with the Instantsearch.js library.
You can take a look at it here : https://github.com/MKCG/algolia-on-redisearch/tree/master/ap...
With the Covid crisis we were no longer able to pay 3K per month for a basic search engine and now we are using a solution based on this on production (> 30M search/month).