The right move depends on the size of the cheque

The amount you have to invest changes which strategy makes sense — not because the principles differ, but because costs, diversification and flexibility scale differently at each level. With $100 a month, the priority is simply getting started cheaply: a single broad index fund through a low-cost savings plan, where fixed fees would otherwise devour returns. The habit matters more than the holdings at this stage.

As the figure grows, the questions multiply. Around $10,000 you can think seriously about asset allocation between equities and bonds, and whether to deploy the money all at once or spread it over months. Lump-sum investing has historically beaten averaging in roughly two-thirds of periods because markets rise more often than they fall — yet averaging in can be the psychologically sustainable choice that keeps you invested at all.

• $100–$1,000 — start with one broad, low-cost fund and automate it.
• $10,000 — formalize an allocation and decide between lump sum and phasing in.
• $50,000–$100,000 — consider an emergency buffer, tax efficiency and diversifying across accounts.

At larger sums, what protects you is structure rather than cleverness: a cash reserve for emergencies, tax-aware account choices, and a clear separation between money for the next few years and money for the next few decades. The strategy that suits $100,000 would be overkill at $100 — and the simplicity that works at $100 would leave real money on the table at $100,000.