Just did a 15 minute session now.
After just 15 minutes. Only cars I had already were a plain peyote and a tornado. So that's 5 x gang peyote (sells for 13k).
if driving there with a peyote (kind of have to follow a particular route, but just experiment. there's another spawn location around the corner. once you do find a good route just do it again for repeatable results):
shortcut back to the garage. Takes about 18 seconds.
The garage in question, below:
After 4-5 peyotes you finish the mission for:
83k in 15 minutes. There's a ls customs just down the street. If you don't want to wait the 48 minutes between selling them you can kill the gta5 process from task manager right after selling, and relog. With the loading times I don't bother. Plus that's probably not kosher. Just sell whenever.
final note: it's probably the best secondary garage anyway (close to airport for pegasus access, close to ls customs, etc).