Investing $ 100 a month is a good solution. But you have to understand that in a short time you may not make a profit. It may take longer. But it is still profitable to invest now.
The guy, sure, can invest 100 dollars or any other sum in the cryptocurrencies every month, but I do not see any sense doing it. Sometimes, alts jump high or grow. Depending on the situation, you can sell them or spend more money to buy tokens when they are cheap.