How much would you charge a customer for a single tenant server of a SaaS platform and associated devops support essentially a network level split of our service for a specific customer in addition to the normal pricing?

It depends on pricing and how much work it takes on your side. If you have everything spread across many machines already internally and you'll just route this 1 specific customer to this 1 specific machine, it should be easy. If it requires redoing deployment and automation, monitoring etc. then it might be a lot.