I guess it depends on what your goals are. When I was building https://spoilerfree.gg last summer I was able to get enough data to figure out where users were falling off the site for about $100 of Reddit ads. I think I got over 1000 users to come through the site.

I feel like ads before you have product market fit are useful for learning what is and isn't working (although they are inferior to user interviews). After product market fit they become useful for driving revenue/adoption.