I highly doubt that when you rent out the Alamodome for a smaller config, you are paying for the whole stadium. The booking fee is likely dependent on the configuration setup you are going to run.
I'd guess like you, that it's likely cheaper to rent a venue like they did for Slammy and BFG as opposed to the the Alamodome but we have nothing to prove that. It seems as such but everything is not always as it seems. Hard to be sure here. But I think the overall sound of the fans in attendance would be better captured inside a smaller venue (unless they can fill this Lockdown venue up), which is important IMO to those watching on PPV and for the DVD.