“I'd say only gamble money you can afford to lose maybe it's going to come back but maybe not and even if it comes back it may not come back in a week so can you actually afford to finance this or maybe you need to change a bunch of things before you can actually get there.”

Cash availability is as important as the return goal — only spend money you can afford not to see for a year

Return on ad spend is not a cash-flow-neutral concept: you spend today and get revenue back over weeks, months, or years. Cash availability is its own separate check distinct from the return goal. A bootstrapped founder with $30K in the bank cannot afford 12-month payback cycles even if the math works in theory — the business starves waiting for money to return.