It's $100 for a 5 year membership, which you can renew (TSA Precheck alone is $85 with their new program; so if you're going to get it, you might as well get Global Entry for the extra $15 to get expedited arrival back into the U.S. from abroad too...this has all ready saved me hours).

I find PreCheck useful, although the lines are getting longer because airlines are now allowed to add their most loyal Frequent Fliers without the interview and known traveler number. In full Precheck lines: belts, shoes, and a light jacket may remain on you plus liquids and laptops can remain in your bags. You also will not go through a body scanner with Precheck. Yes, it is somewhat random so you are never guaranteed Precheck for each flight at airports where it is offered (and in a few cases, the Precheck line is only open for a few hours), but it appears on here that you are much more likely to get it if you have Global Entry. Precheck has been expanding to more airports lately, so it is only going to grow.

After applying online, it takes about three days to a week (although it took me several weeks to get a conditional approval response). Appointments at an enrollment centers totally depend on the location and availability. For Philly, it was completely booked with zero spots open for over six months on the schedule when I was given conditional approval but a lot of slots were open in San Antonio (where I also spend a lot of my time) so I just got it there the day after I arrived back in San Antonio one trip.

One tip is even after you schedule your interview, keep checking regularly online for cancellations because spots will open up and you should be able to get in sooner if your scheduled appointment is much later. "Walk-ins" are sometimes accepted in certain locations and are denied in others. It depends on the office and officers who are working that day. You can search this thread to find out more about the SFO enrollment center.