EuryDace has a very nice guide, but for people like me who has a bit of trouble holding a clean drift around circles for awhile without hitting a cop or messing it up, this wasn't doing it for me very well.
Here's what I ended up doing and it actually seemed pretty fast and easy, it nets in around at least 100k every 3 minutes if you do it right (shouldn't be difficult) I won't claim it's the best method but I believe it is a pretty good one (imo) and worked good for me.
Sorry in advance for the long reading!
*THIS IS BEST DONE WITH A FAST CAR THAT CAN HIT 0-60 VERY FAST*
What I did is from the garage, go left around the corner and straight down the road, over the bridge (Los Sol Bridge) and to the left should be a police station (if a cop is there, you're in luck!) If you're still having trouble finding it, look just left of the garage for "Macdermid's MileMile(Hard)" on the map. I kinda show the location at the end of the video below.
This is where it gets fun, you don't absolutely need a cop but it makes it faster. From the top of the bridge, sprint to the other end just past the police station (or until you see the "high speed" multiplier) then hit the e-brake to do a 180 (try not to over turn for more) and turn around (so you get a "drifting" multiplier). Come to a complete stop so you can start to get another 0-60 multiplier and speed off back to the bridge until you see another "high speed" multiplier then e-brake back around, get the drifting multiplier again, rinse and repeat over and over. Btw, most of these points come from the 0-60 multiplier so get it when you can!
EDIT: Thank you TONI III04 for this tip. If you want another multiplier, go for the "Perfect Nitrous" multiplier. This is best done with either the nos tuning set to full power or having a low capacity tank (or both options)
I didn't think of this cause when I did this I had a lot of duration on mine and it didn't drain by the time I needed to turn around so make sure you don't have a long Nos duration for this like I did.
With a cop you can get even more multiplier to help and possibly keep the score from ending and if you can get any near misses, go for it! But don't hit civilian cars or they will just be obstacles in the way. From my experience, you can hit a cop as fast as you can go and you wont crash so smash them if you want to for another multiplier if you want. Keep this up and do it right and you'll be getting at least 100k in 3 minutes easily.
If you start getting a lot of cops like, lets say.. 5+ and they block the road a lot, run for it and come back later, you don't want to get busted or the fines will be pretty hefty but if you escape then that's money for you to get.
For me, I used a maxed out GTR Premium with extreme grip tires but had tuning around 30% to the left for a bit of drift. This car (at least mine) has 1233hp and 0-60 in 1.67s, but you can use whatever as long as you have a fast car.
By the time I needed to grind to 70, I was rep level 55 or so and had already done all events, so this can be waited on for awhile, while you go through the story and you'll get good cars by then anyways so money wouldn't be bad. Don't forget to do dailies!
This is a little clip I made to kind of give an idea of what i'm talking about, plus some people prefer video (this is my first guide btw)