It's possible, but that's a weird requirement (weird you'd want all your clients on the same repo, anyway) and you'd be able to circumvent this and come out pretty swell on the other side if that money is really worth it to you.
Why would they not use the same repo? It can easily be a standard product - look at the example further downthread of the unreal engine: All clients get access to the code. 2 private repos and 90657 users.
That's an extreme example, but we also have a single repo that a lot of collaborators get access to.
