Sources for Data
1: On June 6th 2016 Sound Transit Planners presented a plan to the Expert Review Panel where they showed that total boardings in 2040 without ST3 would be between 400K-491K and that with ST3, boardings in 2040 would be between 561K-695K. See “Total Boardings”
Assuming the larger numbers, the difference between 491K (no ST3) and 695K (with ST3) = 204K new boardings with ST3.
If you divide 204K by the total cost of ST3 ($54 billion dollars) the cost per boarding = $264,000. Since most riders do two rides a day (in and out), this means the cost per new rider = $529,000.
2: An average family with two cars worth $45K, living or renting in a house worth $515K, and with a household income of $71K will see their Sound Transit taxes go from $380 to an addition $620 for a total of $1000.
4: The Puget Sound Regional Council published a model run mode share that states that in 2040 there will be a total of 18.9 million daily rides a day (car, walk, bike, transit, etc.) in the Puget Sound Area. If you divide 204K (number of new rides with ST3) by 18.9 million total rides, it shows that the new daily rides in ST3 will amount to 1.08% of all daily rides.