Who enforces the price? I can just advertise any address I want with BGP.

If the address space wasn't assigned to you via a RIR, it won't take long for someone to notice though and get your announcement by enough relevant large tier 1 & tier 2 ISPs that your announcement won't reach most of the Internet. At that point you can of course use the addresses locally, but you can't reach the rest of the internet.

This type of squatting was somewhat easier in the past when less of the IPv4 space was actually in use and when less strict filtering (or RPKI) was in place. Still happens, but it's getting harder and harder.

and be promptly kicked off the internet by your isp... or somebody elses.

Unless you own routers that other ISPs almost depend on, then you can say "take it or leave it"

So lets say Centurylink decide to start advertising blocks they don't own, people like Cogent and Tata drop their links, people with multiple ISPs drop their links (shifting traffic to other tier 1 ISPs), and those with centurylink only change their ISP because they can no longer get to facebook.

That would work if most people had multiple ISPs, but most don't actually. Thus, you have some bargaining power, especially if you have some network advantage or feature nobody else has.

If your local ISP advertises the wrong prefixes they get cut off by transit and peers and their customers complain they can’t get to any sites.

Then that ISP can take it or leave it if their customers are complaining.

The customers dont get internet access, they don’t pay, crappy ISP goes bust.

I'm guessing that crappy ISP is the one that wants to disconnect from my network.

If you are leaking routes you shouldn’t then a decent ISP will disconnect your crappy network.

It happens a lot, usually because of mistakes, it’s typically fixed quickly.

Feel free to null route or redirect any IP you want in your network. Just don’t be surprised when other networks cut you off if you advertise those routes out of your network, and don’t be surprised when your customers are unhappy because you aren’t transporting the packet to the target IP as is your job.

If you really believe what you’re saying, which I find hard to believe myself, I recommend you try doing what you’re saying and report back the results.