Hello Artie,
Survey123 for ArcGIS is free to use. However, it does require an ArcGIS Online organization (or Portal) to host and secure your surveys. If you already have an ArcGIS Online organization, there are no additional costs to begin using Survey123. If you do not yet have an account, the link above has instructions that will walk you through the creation of a free trial account in ArcGIS Online. You can use this trial to "kick the tires" and see what Survey123 has to offer.
Let me know if you have other questions that aren't included in the link above!
Thanks,
Scott
EDIT: Hosting of surveys in ArcGIS Online uses the credit model outlined here (under Service>Storage as feature services).
