So also when the market is down to some extent, the person with $30 may likely loss more because of the money is small. Blockchain has the minimum amount one can invest if am not mistaken but if your amount is too small may be difficult to make easy progress.
I disagree that the lower the capital in your investment, the more the loss. Because the greater your profit, the greater your loss if the price falls in the opposite direction.
For example, if you have $100 in capital and profit $30 from a 30% increase in price, you will also lose $30 if the price falls 30%. So, if your capital is $30, you will undoubtedly gain less if it rises, and you will lose less if it falls. Based on my experience with Binance, the minimum trade size is $10USDT. And $30 is more than the bare minimum. I'm not sure what the minimum is on other exchanges.
So, depending on your vision and ambitions, $30 can be a suitable starting point for investing. Some people take it as a challenge in the hopes of increasing their investment if it succeeds.