This is just my two cents. I'm the one who used to be using youtube ads. Unless you have the money to burn, you're ok. But, if you want to really improve your channel organically for the long term, dont use youtube ads, instead, optimize your youtube SEO in every aspect and of course the contents itself (I'm still learning on this too).
Answering your question, from what I used to get per 7 dollar is about 1000-1600 views and about 15-21 subs, but is not always the same. Btw, you know that watch hours from youtube ads doesn't count up for YPP eligibility right ? In case you didn't know about it.