Spend where you act
Upgrade the tool you touch every minute. A better drill or weapon pays back on every single fight you take, so it is never a wasted coin.
The character database shows who benefits, and the boss database shows what you must beat to earn the materials for the next tier.
Economy first
Most upgrades wait on resources. Build the income before the luxury, or you stall the run and watch stronger foes arrive while you sit poor.
Do not over-buy
A maxed early tool beats a half-built late one. Finish a line before starting another so you always have a payoff you can feel in the field.
Check the cast
The characters page shows who gains from which gear across the series. That helps you pick the right early upgrade instead of a dead end.
Time the spend
Buy when you have the surplus, not when you are broke. A planned purchase between waves beats a panic buy that empties your coffers.