My gut reaction to what I’d pay for Twitter is “nothing.” In general most services that would even compare to this service are free.
When you look at the possibilities of what Twitter could offer above the basic level of service that they currently provide you only need to look as far as all of the sites, services and tools that have been developed using Twitter’s API. So if Twitter could figure out a way to provide services to brands that would make it easier to interact with those that follow their brand and increase the depth and value of that relationship I’d pay more.
We’ll have to see what comes up next.