The equipment stack
Tank + stand + filter + heater + light + test kit + conditioner + substrate + decor: the "20-gallon starter kit" doubles its price before the first fish. Budget the stack honestly ($150–300 for a basic 20-gallon system) and the hobby starts with margin instead of corner-cutting.
The monthly line
Electricity (heater + filter + light: $3–8/month), consumables (conditioner, food, test reagents: $5–10), and the occasional replacement (thermometers, impellers). A healthy 20-gallon runs $10–20/month — trivial next to the fish lives it purchases stability for.
Where not to economize
The test kit (the tank's dashboard), the conditioner, and the heater thermostat — the three cheapest items with the highest failure costs. The decor and the fancy light can wait; the water chemistry cannot.