What's a normal rate for selling posts on my 3 million follower FB page?

Need some direction here, since I don't know what I'm doing.

I've run a FB page for years that grew & grew to 3+ million followers. It still grows at 10-40k per week. A few years ago, engagement was incredible, but it suddenly dropped the second I started posting links back to my own website.

Now, it's the usual terrible engagement rates that most people get, which is 0.5%. Sometimes I hit a home run and get a 10% response. Either way, I don't know what I'm doing, and gave up hope of it ever being worth anything. Just a hobby/labor of love.

For years I've ignored emails & messages from people asking to advertise...or for sponsored posts, collaborations, etc. No time. I have a day job & I have a life. Although, I've tried posting stories for Little Things, AWM, and MyLikes...with limited success. LT sometimes did ok a year or two ago. Once I hit the $800 mark in one month with them, but in the last year, the numbers have dropped.

Almost every day some new revenue sharing company contacts me to do the same thing. However, lately I've been talking to the other companies that offer a flat payout for stories posted. Most ask for $25/post, but I've seen pages with 1 million followers offer sponsored posts for $50 each. So...I should ask for $150?

Strangely, there seems to be tons of such firms/sites out of eastern Europe.... I have sold posts to a few of them, and I'm trying to get an idea as to what I ought to be charging with a 3 million follower page. I don't know if I'm getting fleeced or not.

You know the kinds of article/link bait sort of websites, right? There's a million such sites whose strategies are to simply drive traffic via FB, and nothing else, and then optimize for tons of ads. A type of arbitrage. I have nothing against these types of sites; I'm sure some of you reading this make your living this way.

Here's my issue: recently I keep getting approached by reps from one such company for their site which is in the Alexa top 1000, and they are offering me CRAZY numbers. They're offering $500 per post & want to buy 4 per day. I don't want to say their name at this point.

I keep trying to act calm & ask questions...and they don't really say much...but I'm trying to get them to slip up; I want to figure out if they're for real. I don't want to be blinded by the $$. Is this too good to be true?

Is this for real? Are there any risks here? I mean, if I get a 2 grand payment every day via PayPal, am I pretty much fine? I'm not a criminal, so I can't think of the inventive ways that I could be cheated. What if I post for them for a month, and then they open up a formal complaint against me (to get their money back) as some kind of tactic to get posts/traffic for free? Are there any horror stories out there?

And what's the normal sponsored post rate for big FB pages? From what I can tell, niche doesn't matter...or does it? And historical engagement rates also doesn't seem to matter! Some of these firms tell me it's up to them to make sure their articles are engaging....