We did use software escrow at my last company (hospital customers). I believe we just did source code hosting and it was roughly $1000 to set up and for the first X customers, and additional customers were a smaller additional fee. Given how aggressively SaaS services are priced these days, companies like Iron Mountain may have special pricing for SaaS. Getting a quote is the best way to go!
Also, if it's too expensive to do for everyone, you can of course offer it as a premium feature or a differentiator for an enterprise pricing tier.
Also, if it's too expensive to do for everyone, you can of course offer it as a premium feature or a differentiator for an enterprise pricing tier.