When using ODK Aggregate on Google App Engine (GAE) it was possible to use their free tier if you had low usage. In February 2019, ODK Aggregate v2.x was released with support for GAE removed, because GAE has become more difficult and more expensive. More details in this post.
Web infrastructure generally costs money. Central was designed from early on to be stable, predictable, and fast on limited hardware. It can perform well on the second-cheapest DigitalOcean configuration at time of writing ($10/month, 2GB memory). All ODK software is open source and free to install and use, unfortunately it's not possible to also offer services like web hosting for free.
Other options include:
- running Central on a local hardware/server
- pull forms from Collect using Briefcase
- connect Collect to a Google Drive account and send data to a Google Sheet (PLEASE NOTE: Collect will stop connecting to Google Drive/Sheets in Fall 2023)
- KoBo Toolbox is part of the wider ODK ecosystem (it's built on ODK source code) and they currently offer free accounts on https://www.kobotoolbox.org/