In your opinion, what is the cheapest current method to get real targeted followers in a specific niche?
It’s been costing me around $0.30-$0.50 per follower using Mass DM services, but I want to get it down much cheaper to $0.20 per follower.
Been considering ads, but thought that would be too expensive
Any creative or effective methods?
Try changing the CTA.
DMs are just a method of transportation for the CTA. Ads are a method of transportation where the social platforms own the bus.
I would say for you to maybe start using Jarvee in the Premium account, get the unlimited LE license, and use that to improve your engagement per post. Other things can be done as well, but at least this would lower your costs significantly, mainly if you can automate the creation of the IG accounts and buy some gmail accounts to register them.
Your ROI will most likely depends on your account quality. We grow some account at a 0.05$/follower rate, while some others have issue doing better than 0.30 or 0.40$/follower.
You can’t compare the growth of a beautiful model Versus a bottle of water company… 
Strategy / Niche / Content - Creativity