Looks like your best total value is 1-stop YVR-BOS. Seattle doesn't seem to generate the cost savings you'd need to justify the extra 6-8 hours of time. BOS vs. PVD, however, does save a couple hundred bucks and doesn't have too bad of a time-cost associated with it.
Even Southwest out of Seattle is showing high fares. I'm guessing it's just a case of two sets of cities that are very, very popular for summer travel. I routinely used to pay $750 R/T for MCI-SEA in the summer months and I know YVR wasn't ever any cheaper...
Pricing out into the winter months shows $300 R/T's for SEA-BOS.