Last Thursday, Linden Lab announced they were lowering the price, and offering two levels of Event Regions. The lower priced one was called "Event Region Pro." It cost $449 USD a month "with no additional setup fee." They have the 175 avatar limit, more than 20% improvement in script performance, and upon request extended chat range. Three more options were available for fees. An increase to 30,000 Land Impact would cost an extra $30 USD a month. An Instancing Service, or cloning of an Event Region, was $50 USD per request. And rollback services, done within 48 hours, were $25 USD each.
The more expensive level is "Event Regions Elite." Besides having the features of Pro, the Land Impact is raised to 30,000 with no fee. Niether are there fees for rollbacks or cloning of event regions, and "White-glove concierge service." The price is "$599 per month with no setup fee."
It would then have this to say about region rollbacks:
In addition to these new product offerings, we are pleased to announce region rollbacks as a service. This service is now available to all private region owners. If you make a mistake with region content, or want to restore a previous build, you can now request to roll the region back to a specific time. Please note that rollbacks will still be at Linden Lab’s discretion and are not guaranteed. Rollback services are available at $25 per request.
A rollback of a sim was once done without a fee, but at the discretion of Linden Lab, which cautioned to treat them as "a safety mechanism to be used only when something goes very badly wrong," and one could only roll back a few days.
For more information, check the page on the Linden blog.