Field Box 40 is expensive but there are so many "extras" included in your ticket price. My favorite was probably singing 'Sweet Caroline' with 40,000 strangers. They use to only play the song during the 8th inning of games the Red Sox were winning, now they play it at every home game during the 8th inning.
Sitting behind home plate is great. You'll have a whole new outlook on how talented you have to be to get a hit as fast as those balls are thrown. Your view of the Triangle in the outfield will be pretty minimal but that's going to be the same story at all of the other ballparks when you sit behind homeplate.

If you have some time to kill in Boston, take one of the three Fenway Park tours; The Sunday Premium Tour and Batting Practice Tour are the two best. Both allow you to go down on the field and are only $20. Only $20 to walk the same field Babe Ruth and Ted Williams once walked on? It's a once in a lifetime experience.