Why not buy one bitcoin today? If it falls to $600 you buy one, at $500 you buy one. And so on. Your average buy would be ~$ 633. So you don't have that much lost(%). If you bought one bitcoin today and it wil start to rise to $2,000 you don't regret. If it keeps falling you can buy with fiat to make your average buy <<.
Just make a plan. And if you believe in Bitcoin this is the best investment strategy.
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Making average at every fall of Bitcoin is good, at present I don't have money. I wish to invest in Bitcoin in systematic investment plan monthly. That is common wish of every people becoming millionaire by over night. But it is fate for some people only, most of the people are not invested due to fear/risk so they have not benefited.