You're overestimating the cost, but underestimating the time

The project cost $346 million and took 6.5 years from the start of construction in 2016

But that's a bit misleading because the main component of the project was a sewer / drainage / utility replacement. All the subsurface infrastructure on Van Ness was like 150 years old and falling apart.

The transit-improvement component of the budget added up to $169.6 million

edit to add some additional stats:

https://www.sfchronicle.com/sf/article/S-F-s-Van-Ness-BRT-created-a-ridership-boom-17556984.php

The 49 line had about 740,000 total boardings in September, up from 549,000 in March, the month before the BRT project [opened]

(Divided by 30 and this adds up to ~24,500 boardings per day...relevant for people incorrectly pointing out that this bus looks "empty")

https://www.sfexaminer.com/archives/van-ness-buses-show-big-speed-reliability-improvements/article_9b9b7e2e-510e-59a2-ba68-95e965870d99.html

Travel-time savings were as high as 35% northbound, or nine minutes per trip, and 22% southbound, compared to a 2016 baseline that predates the recent construction disruption on the boulevard.

Travel-time variability decreased by up to 26% northbound and 55% southbound, according to the data.

[T]he lanes appear to have encouraged more people to take the bus, with ridership increasing 13% in the first week the Bus Rapid Transit lanes were in service, a pattern that has remained consistent in the following weeks.

Another factor driving up ridership could be improved bus frequency made possible by quicker travel times. “A 20% reduction in delay means a 20% increase in capacity and allows for a 20% improvement in frequency, because we can turn the buses around much more frequently,” Tumlin said. “We have already taken advantage of that in order to improve the frequency on the 49.”

Here's a couple YouTube videos going in depth about the design and performance of the Van Ness bus lanes:

KPIX 5 News: San Francisco's new bus rapid transit lanes on Van Ness getting rave reviews

MARTA: In-depth Study: The Van Ness BRT Corridor - San Francisco

It's important to note that I took this earlier this afternoon, on a Sunday, at about 2PM. So bus ridership is much lower than a weekday or peak hours. There were probably about 20 people on the bus, instead of the usual weekday 50-150. (The bus comes every 5-7 minutes, so on a weekday that's thousands of people per hour getting shuttled down this corridor in a high-efficiency lane.)

Usually, vehicle traffic would be light on a Sunday too, but there's a Cherry Blossom Festival parade passing by a few blocks ahead of where I shot this. Vehicle traffic is not typically seized to a standstill even during peak hours on Van Ness, but it is often very congested regardless. And when it is, this bus lane is a godsend.