I don't think anything gives you as quick, and as high % of a return as AM does.
At least for lower amounts <$a mill or some shit.
Anytime you can earn money on the money you aren't going to be using
AND beat the inflation rate, it's a good investment.
That isn't the same as money you have that you
will be using. If you know you are going to be spending $20,000 a week on PPC to make a 20-40% ROI and you have weekly payouts, you know you now need to have at least $40,000 in the bank to do business with. If you factor in the occasional negative return on the campaign, networks not giving you as big a payout for whatever reason than you expected, etc. Then you probably need to keep an extra $10,000-$20,000 sitting idle in your bank.
With the above in mind and following the same example, you know in a month you'll have another $20,000 to invest or keep idle to do other stuff with it. At some point there isn't enough money on hand to continue
"investing in yourself", so it makes perfect sense to keep investing in other residual markets to be productive with your money.