Cost is absolutely a valid reason to abstain from having children, but it's just one of many; and is a blip on the radar compared to things like refusing to subjugate them to the dystopian hellscape of a planet we're building for them.

Choosing to have a child is an act of cruelty, usually made as a result of completely selfish thinking like giving yourself / your partner that 'little bundle of joy' or continuing a family legacy or even to build a footsoldier to stand as fodder against the hordes of people pouring fuel into the dumpsterfire we live in.

...but yeah, they are also expensive.