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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.