SSO isn't a tax. You either need a single method to disable an account across all providers instantly and enforce password policies globally, or you don't. Do the risk vs reward math and then put the line item in your budget. Get a discount or use a reseller to avoid retail.
It’s really a cost/time to market comparison between the SaaS and you just using SharePoint or managing some substitute. If you’re proposing to drop $80 u/m for Airtable, there better be a tight business case.