Multisite registration after payment

Hi, currently I'm trying to setup a new service. It should be possible for everyone to signup on this service. But their site/account will only be created after their payment is completed.

So it's much like edublogs.org with the main difference that there is no free level. You have to pay directly in order to get into your account.

I have tried to set this up with Pro sites, but I just can't manage to complete this setup. So I'm hoping you guys could help me out.