Years ago I worked on the building of the Interstate 205 Glenn Jackson Bridge, and if my mind serves me correctly I was told then that the bridge was built with light rail in mind. If so, light rail maybe could be added with very little retrofitting.
The bridge is already high enough so you wouldn't have to raise it for clearance of water traffic. Light rail already runs to the Portland airport so I would think being that close would cut the cost of tying it into the bridge.
I feel that it may be a lot cheaper to add light rail to the I-205 bridge and run the line from there into Vancouver. Use the money saved from building light rail with the Columbia River Crossing to fund it all. Also, that would save the cost of a new I-5 bridge tolls, which none of us can afford.