Set a monthly hobby budget.
Hobby budgets hold up when they are set as a percentage of take-home pay rather than as a fixed number, because the percentage scales with your circumstances. Five percent is a common guide for discretionary interests. The refinement that makes it survive contact with reality is a sinking fund: reserving one twelfth of a planned large purchase each month, so the expensive item does not devour a single month's allowance.
Monthly pool
Monthly hobby budget = take-home pay x hobby share
Allowance after the sinking fund
Per hobby = (monthly pool - annual big purchase / 12) / number of hobbies
It is a common starting point for discretionary spending, but the honest test is whether savings, debt payments and essentials are already covered. If they are not, the hobby share comes second.
Because a four-hundred-unit purchase does not fit in a one-hundred-and-sixty-unit month. Spreading it over twelve months turns an unaffordable spike into a planned cost.