Depends on the product. I'd never guarantee 3 nines (a bit under 9h of downtime per year) to my customers of any product with moderate complexity without multicloud(except for a cloud provider issues that affect entire region).
On the subject of cost. Multicloud doesn't have to be expensive, but it requires foresight during the initial design not to lock yourself up in proprietary tech.
Things such as AWS cognito, lambda elastic beanstalk, SQS can make it completely uneconomical to do multicloud.
But let's say you run roughly equivalent services (load balances, DNS, vms, containers for example under GKE, s3 compatible object storage, etc) it doesn't have to cost an arm and a leg.
