That's such a good question, I find most people (myself included) being clueless when it comes to this.
I have approached influencers in the past and have offered $100 - $500 for accounts who have between 100k and 500k+ followers. I've been asked for much more but I tend to pass on those requests as I honestly can't justify the price.
The trick is to find influencers in the niche you are advertising and have an incentive prepared for new ppl to join. It almost never worked for me when influencers try to sell a product directly, but when they introduce a freebie, I got an average of 100-300 new ppl joining my mailing list from this shout- outs.
However, they will have to leave the promotion posts/videos for at least 3months. If the posts get deleted within 24h (like IG stories), I find it a complete waste of money...
Would love to hear other experiences and maybe we can grow less clueless together 😆