I know I've suggested this before, but I thought I'd bring it up again in the context of the Crenshaw Line Northern Extension having Metro-produced alternatives. I believe running the line up La Brea is crucial to having an optimal long-term metro network, as ultimately, and east-west line must also be built on Santa Monica, and the other alternatives would interface weirdly with that while taking a much longer time to get from South LA to Hollywood. Therefore, I decided to see how much of a Santa Monica line could be built with the 1.7 billion dollar difference in price between the La Brea and Fairfax lines.
As it turns out, a line crossing the La Brea line from the east and continuing west, underground, via Santa Monica/Fairfax to Santa Monica/La Cienega could be built, as well as a connecting track from this line to the northbound stretch of the La Brea line would cost exactly 1.7 billion dollars. This would allow trains to operate from LAX to Hollywood/Highland via La Brea, turn back along the track to a lower level of Santa Monica/La Brea and west to Santa Monica/La Cienega. The train would then turn back to Hollywood/Highland and then head south back towards LAX. Eventually, the West Hollywood Branch could be extended southwest to Beverly Grove and Culver City, and east to Los Feliz and Downtown. This would create an ideal gridded system of lines between DTLA and Santa Monica, thus serving the most area and keeping journey times short. While this branch would not quite reach the Beverly Grove and West Hollywood Park areas in its initial iteration (each would be about a 10 minute walk from Wilshire/La Cienega and Santa Monica/La Cienega, respectively), the main east-west corridor of West Hollywood would be served, and a metro station would be within walking distance of the Sunset Strip.
Moreover, ridership numbers would be higher than any alternative Metro is currently studying. I estimate the La Brea alternative plus the West Hollywood branch would have a daily ridership around 120,000 people, 29,200 more than the San Vicente/La Cienega alternatives, and 32,800 more than the La Brea alternative. This works out to a cost per rider of $39,167 (compared to $52,988 for Fairfax; $48,458 for La Cienega, $47,357 for San Vicente, $46,332 for Vermont, and $34,404 for La Brea). Assuming Metro can pay the extra 1.7 billion, I think it's worth it. It is also worth noting that to underground the La Brea line the whole way would cost exactly 1 billion more, and might also be a good use of the money, as it would make the corridor nicer and cut about 3 minutes off the 12.4 minute travel time by straightening the sections in Mid-City. However, this would only be warranted with denser development. Come to think of it, Metro could upzone the La Brea corridor and implement enough value-capture to finance this extra 1 billion.
Below is a map of my proposal: