If your going to a Red Sox game this year, you could consider parking at the Prudential. They charge $16, which is one of the lowest parking price of any garage around the Fenway area. I believe the garage at 100 Clarendon is slightly cheaper, but adds about 10 minute walking time to/from Fenway.
According to the signs around the Prudential Mall, the walk to Fenway is only 10-12 minutes. However, using Google Maps, the walk appears to be about 20 minutes long. I guess it all depends on how fast one walks.
If you take the Mass Pike, then the parking at the Prudential is a good option, its really easy to get to the garage off the turnpike, and even easier to get back on the turnpike. After the game, instead of getting on the Pike at Copley, take the Mass Ave. exit. It's a little less traffic to maneuver around.