For RIPE (don't know others) the are two ways: you can either sign up as a full member (an ISP) for 1500€/year, which gives you the same rights as any other ISP. You can also request a "provider independent" or PI address block, which comes with some contractual restrictions (you have to use it yourself and you can't act as an ISP), from a member for 50€/year plus their profit margin. Officially you should get one from your actual ISP, but there are a few RIPE members who sell easy access to PI blocks as part of their business model.

the linked page has an earlier blog entry talking about that:

https://blog.lyc8503.net/en/post/asn-1-asn-registration/

quickly skimming the article i couldn't see a specific price for the ipv4 block, but ipv6 is cheap - the article mentions having to pay at least $50 a year + service fees to a "LIR", and you also need a BGP-enabled hosting provider which i imagine will come with similar cost at least (don't quote me on that).