I called Strange Brew in Marlborough yesterday and thought I would report what I found for the betterment of beermanity. Their exchange program requires a "buy in" with the purchase of a full 5lb tank for $130, and then the cost for a swap is always $5 cheaper than the general rate. The current 5 lb exchange rates are $20 if you're in the program and $25 otherwise.
A new, full 5 lb tank is $100 without buying into the exchange program.
I also checked with Homebrew Emporium in Cambridge last week and they were charging $100 for a 5 lb tank, and ~$26 for an exchange. I think it was $26.95, but I can't remember exactly.