Dynamic IPs are not actually that dynamic. In this day and age of routers and cable modems, you keep the same dynamic IP addreses unless you leave the power off on your router / cable modem at the time the lease on the IP address runs out. Depending on the ISP's config the lease can last for days or hours. I personally have had the same Dynamic IP for well over a year. If you are on old skool dialup then you normally got a different IP every time you dialed in but in most cases now the same ip address shared by many people in the same day / week doe snot happen anywhere near as frequently as it used to.
As for ISPs having to pay for IP addresses, well actually no they dont. There is an administration fee for each new assignment and there is the time taken to fill out the RIPE form showing justification for why a tiny company needs 10 million IP addresses (mainly to try and make the dwindling pool of IPv4 addresses last) but there is no X pound per IP fee imposed onto ISPs
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