Too bad, cause if you know the trick, they spawn reliably. I always replace the one I sell and can keep myself well stocked
. Between that and Trash Talk/A Titan of a Job/Denial of Service (Titan gives you 18k min. the other two anywhere between 13/14-18/20k depending on how long you take), I don't worry about money anymore. Plus if you just start doing missions with friends for a while you'll find money accrues quickly.
I'll say if you play first person though, I found out the hard way, you have to be in third person (and you have to drive up the road a few blocks in third person... you can't be close in first person at all or it won't work and you'll have to drive to the "reset point" and do it over again in third person). And you already have to have one of the chrome dubstas (you can mod it and change it after you get it but you have to have one of them, not one of the regular ones you see driving around).
If you ever see me on, I"m usually hanging out in an invite only or crew session so try to join me and if I'm not doing missions I'll happily show you the trick (I was frustrated for a while too but for me it turns out what I was doing wrong was being in first person). I'll even grab you a dubsta to start out with if you don't have one (I recommend you insure it and keep it to spawn more with).