To extend on Mr Mus2nd's solution - those with TBoGT DLC can get this done in, literally 20-30 minutes.
What you need to do is complete the mission entitled "Momma". Very early in the DLC, where the debt collector is hassling Luis' mother.
You will follow said debt collector out of the house and around the corner, down an alley and into a room where the LC fights take place.
You will be forced to fight three opponents in order to earn money for the debt collector - as he will bet on you winning the first two fights, and losing on purpose in the third.
Complete the mission - it'll take 5 minutes. Then head back around to your safehouse and save your game (just to be sure)
Once you are done the LC Fight venue is unlocked for you to visit whenever you want. Go in and pay $50 to spectate.
Now, every time a fight is about to start, you'll be asked to bet on one of the fighters. This is where the easy money comes into play.
Look at the fighters. Is one of them tough looking, doing a mean impression of Tyler Durden (Brad Pitt style) from Fight Club? Is the other fighter a bit, well, doughy around the middle? Is he dressed in a suit? Does he look like he could do with a good feed?
If so, bet on Mr Durden. And when I say bet, I mean bet everything you have. Watch the figures scroll upwards as you increase your bet.
Then, lay your bet, sit back, grab some popcorn and watch the cage fight. If you've successfully predicted which of the fighters will win, you will be treated to a fistful of dollars (apols to Sergio Leone there) entering your pocket.
From there you can do one of two things - dash out, go back to your safe house and save your game (with extra money), or you can bet again on the next fight.
I personally always like to have a look at the fighters in the next fight. If it looks like an easy one to pick a winner in - one guy looking mean and nasty, the other looking like a bit of a weakling - then by all means bet everything again. If you are unsure, just bet $100 - you can afford it!
Continue to bet on winners, save, bet, save and bet again. Eventually you'll be raking in the dough and, after about 6-7 bets, will easily notch up your half million dollars.
Exit the club, save again, and spend your new found wealth on some nice threads for Luis, some lovely weapons - more of those sticky bombs please - or whatever you want.
20-30 minutes to get 55 GS and 80 TA points - not bad at all.