Honestly, monthly ad budgets vary wildly from project to project, some people only spend $50 or $100 a month to understand how the platforms work and see which audience responds best, while others are more confident and invest several hundred dollars or even thousands once their campaign is well-optimized.
The most important thing isn't the amount itself, but that the money you spend helps you learn: which ad attracts the most clicks, which converts best, what time your audience responds best, etc. With a small budget, you can still gather useful data if you manage it well, and once you find an ad or strategy that truly delivers results, then it's worth increasing your spending, because it's essentially nurturing something that's already working.
Everyone spends according to their stage of development. If you're testing, start small, if you've found something profitable, scale up. But there's no magic number that everyone agrees on.