People who finds $20 sufficient and people who don't find $100 sufficient doesn't only depend on plan rather, sometimes it depends on strength of your source of income because what determines how big or small your discretionary income will be is your source of income and I doubt if someone who is interested in Bitcoin investment with strong or good source of income will see $20 as sufficient because it will affect their portfolio I mean there won't be much growth in their portfolio and every serious investors mind or plan is to have an amazing portfolio.
Isn't it better to have a dollar in your pocket than to have nothing? Now if every person wants to invest in Bitcoin with discretionary income, then maybe he will delay it too much or he will never be able to invest in Bitcoin because not everyone has a lot of discretionary income. Everyone wants to invest in Bitcoin to protect their money from inflation and to earn good profits from savings. Since Bitcoin is a long-term investment process, now the question is how can a person make his Bitcoin investment sustainable? To make Bitcoin investment sustainable, he must invest from discretionary income.
For example, suppose a person's discretionary income is $100 per month, then he can invest $70 in Bitcoin from there.
Again, suppose a person's monthly discretionary income is $1000, then he can invest $600 in Bitcoin from there.
Maybe after 10 or 15 years, the Bitcoin portfolio of the person who invested $600 per month will be healthier than the one who invested $70 per month in Bitcoin, but both of them will be able to benefit from it.
Therefore, if there is a reasonable income at the beginning of investing in Bitcoin, without thinking too much, then no matter how small it is, you should start investing using the DCA method.