I have used ads successfully to get from about 0 to 3k followers. I spend about $100 targeting worldwide through facebook because advertisting in tier 1 countries only was to expensive.
What I did was target a broad general interest like painting (if you are a painter for example) and used a $5 a day budget.
Best is to use a good piece of video content and in the description put a simple one liner, a couple of emojis 
and include a CTA like “Follow @xxxxxaccount” so they actually go on to your profile page and check it out.
Overall would I recommend it?
The results can be good and enticing. You can gain some quick followers, especially if you have a good budget and if you want to spend more, you can even target the USA which will bring you very high-quality followers (although it will be a lot more expensive).
But the problem is me and some others on this forum have experienced a lower reach and engagement after dropping ads. Apparently IG tries to lower your organic reach so you will spend more on their ads.
You can read more on that here: