In 2021 Social Media Influencer prices are stable and corporate Social Media budgets are increasing according to the Influencer Marketing Hub: Influencer Marketing Benchmark Report 2021 survey of 5,000 marketing agencies, brands and professionals. SnapChat rates are the same a Instagram but the least likely to follow benchmarks. One [legal advice unquotable] Australian Influencer Agency claimed a 33% price increase for all Influencers with no explanation, I guess they are into marketing.
Price Chart
I collated 2021 prices from influential Australian Influencer agency WinkModels, the Influencer Marketing Hub, Tribe Group and WebFx.

Fees are US$ per post except Snapchat and Blogs which are commonly per post per 1000 unique views. Instagram has a low-high range. This is the prices in table format:

Why Do Fees Vary?
Fees vary due to anything the Influencer and their Agent can dream up but these basically fall into the buckets of engagement, specialisation and workload.
Engagement
Look at the two engagement % numbers below. The smaller follower is more niche so has better engagement than Kim Kardashian.
Specialisation
Different industries charge different rates and everyone claims the magical X factor.
Influencer Workload
Posts are the cheapest. Blogs are more expensive (see above) and videos typically add 25% to 50%. The same travel blogger may charge $1,000 per “static” (no video, no slideshow) post on Instagram and $200 for an Instagram Story post. The service (see below) greatly impacts work load.
Payment Mechanisms
Linking your investment to your impact is done using metrics. Here are the most common:
Pay-per-post: Simple flat rate
Pay-per-click: Based on a call to action in the post
Pay-per-view: Results oriented so popular
Services
Select a package that connects with your strategy, demographics and segmentation. Here is the shopping list of common services:
Shoutouts: Basic promotion using text, images or videos
Giveaways: Sponsor provides gifts
Platform Takeovers: The Influencer drafts your social media account(s) and attracts their followers to your account
Affiliate Marketing: Paying a sales commission to promote
Sponsored Content: Presenting your content.
Product Seeding or Gifts: Gifts in leu of payment. Larger Influencers expect to be paid.
Brand Ambassador: Longer term arrangement
Is ICON a genius step forward?
On the news we have prices such as iron ore, gold and US$. What is your opinion about creating ICON? (Influencer CO$t Number) as the globally recognised influencer price benchmark?

Hi Brian, I really appreciate your blog, it provides strong evidence about payments of influences on mainstream social media platforms such as Instagram and Facebook. One thing I notice is that Influences on Instagram have two sets of payments, partly because of fields they focus on. Services and mechanisms of payment are listed as well, which is comprehensive. Lastly your idea about setting up ICON is fantastic, it triggers me some thoughts about how to do that. Maybe in the future, official institutions will be established to list that and announce daily like stock market. Also, I expect that derivatives around this system will arise.