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You are correct, and this is very important for beginners to know because some of them think that they have to gather enough money before they can start buying bitcoin.
I think it's very subjective and it will vary from person to person. Some people find $20 sufficient to buy bitcoin - while others find $100 insufficient. It all depends on they plans and what they want to achieve - but the market will always be open and welcoming to anyone who wants to buy at any price. I don't mind how they approach buying bitcoin - that's up to them and their plans, but if they want to be long-term holders then they should know that DCA is a good approach.
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.