Seems like the general consensus is $100 to learn the markets, and $1000 to start making real money once you've got the ball rolling. Obviously it all depends on what you can afford anyway, and everyone's circumstances will be different. The current market is very different to the end of 2017, back when a $30 investment in the right ICO could get you 10-50x. Now you're lucky if it hits the exchanges at anywhere close to the ICO price, most are coming in at below 50%. Trading in this market is not for the weak hearted either. I wouldn't advise any newcomers to invest a substantial amount of money until they're fully confident in their abilities, so $100 probably still holds true.