My understanding about EZPass is that you don't get any sort of discount on Massachusetts tolls by using an out of state EZPass (or other device), so if you go directly from BOS to Medford via I-93, your toll would be the same as the standard Sumner Tunnel cash toll of $3.50. As mentioned previously, going the other direction won't cost any tolls unless you come into Boston on Route 1 (and I don't recommend doing that).
Same for other tolls in Massachusetts - you won't get a discount off the regular cash toll amount, your only "savings" is time at the toll booth.