I looked into coworking spaces. The cheapest option, which is basically "you can sit on the couch if there's still space when you get here," is about $500 a month. Boston is unforgiving.
Since I started working from home (almost a year ago) I've found myself missing the in-person professional interaction at times. My wife and four young kids are at home, so there's as much human interaction as I want, but I think it's important to be able to "talk shop" with others in person from time to time.