Again, the real cost is very difficult to determine because of the value of the time spend preparing for the events, and MacRumorsLive in particular takes a lot of time. It's an extreme example because the visitor numbers are so huge; this in turn means that a lot of time needs to be taken to ensure every possible eventuality is thought of and prepared for. We started planning for January's keynote in September (and early discussions about how photos might be included happened way back in March) so there's a long lead-time up to an event where it's being worked on.
Getting someone to implement and run it it on a commercial basis, the hosting cost would be insignificiant compared to the cost of labour.