You need to include the value of your current iPhone.
$90 + $164 + trade in 16GB iPhone 6 for example (~$450 on Craigslist) = $694
Which makes it cheaper to just sell my 6+ on Swappa for $515~ and pay their regular $24/mo. plan. Total spent after the trade-in is $354 ($192 to buyout a 6s+ according to Twitter DMs). Total spent on their non-trade-in plan is $80~ (after selling my existing 6+).