I don’t have numbers, but I’ll answer with an experience from this morning. We’re working on a task (all 4 of us), and we agreed that the functionality will be tested.

Navigator said to write 3 end-to-end tests, and a mob colleague stopped him. We discussed for ~3–5 minutes about type of test for this functionality, and in the end agreed on 1 end-to-end test, and 3 unit tests. I think it’s the best compromise possible for given task.

It took at most 4*5 minutes = 20 minutes of developer time.

Imagine this was done in code-review style. The colleague will write 3 end-to-end tests, and will write them nice, so it will take time. Reviewer will have to make really deep review to spot it could be done in less end-to-end tests. Then will have to explain and argue for this approach. Author will have to rewrite the code. I’m sure that this will take all together hours of developer time.
Or the developer won’t spot it and we’ll end up with many slow end-to-end tests, that would lead to frustration.

Similar situation happens multiple times per day.

So I believe mob saves resources, and produces the best result 4 people can do.