It appears that the valley is about 15 miles long. The pasadena gold line is supposed to cost about $450 million dollars. It aparently would've cost an extra $350 million to put the whole thing underground. Like the pasadena route, the MTA has a right of way cutting through the valley. So I'm guessing it would cost for a valley metro would be similar to the cost of the pasadena line. Which is a fraction of the cost of the plan to extend the freeway.
One of the other reasons cars start failing in an urban environment is after a certain point you start having exponential increases in cost to obtain the necessary right of ways to extend your auto infrastructure.
All I need to do now is verify that the capcities are similar.