Well, I've been just in a couple of years. In 2015 I went to 5 games in Fenway --including opening day-- and a couple more in Yankee Stadium, and this year I bought tickets for 5 games but one was suspended -- vs NYY. I went for my first time as well to a Celtics game in the POs vs Chicago Bulls.
It's a really good experience and I really recommend it... but yeah it is very expensive mostly for me since I travel from Mexico City and we don't go only to the games, we also go to shopping, restaurants, tours, etc in both cities Boston and NY.