Recent example of Back Bay (but extreme) includes an IRS auction for two spots in tandem location in Back Bay in alley behind Comm Ave. These two spots went for over $560,000. for the two of them.
Beacon Hill is not far behind, especially since they have less available.
As far as other area, the prices tend to drop significantly, except for some places in Brookline. This is due to the fact that you are not allowed to park on the street overnight in Brookline, so people with apartments or condos need a place off street to park if they want to have a vehicle.