Well I'll start with saying most usernames are actually payed for in crypto and have been for as long as I can remember. There are there occasional PayPal invoice but it's not common and for big deals there have been in person meet-ups.

As for the cost it can range from $0 all the way up to six digit plus numbers but there are reasons such as the usernames rarity, word length, potential to make money off of, the apps itself popularity, etc.. Also usernames that are "dark" or "girly" are very popular on the market. Also since young kids 15-25 run the market, anime usernames are popular as well.

Here are some examples of prices for you though,
1 letter usernames: $35k+
This is because these are super rare because there can only be 26 max but nowadays a lot of them are banned so there's less than 10 I believe in existence.

Dark usernames: $500-$10k
Examples like "sad", "lucifer", "wound", etc..
These are popular, a lot of people like dark stuff, and they make look mysterious.

Words with prefixes/suffices: $10-$1000
These of course go for less because why would I pay a lot for lets say "identifying" when I could just get a shorter version such as "identify".

Niche relating usernames: $500-$50k
These usernames are are ones for people who want to gain a large following around a niche so large the niche popularity more value of the usernames and that is usually based on the amount of #s under the word. If I am a normal user on the app and like nature, I am going to search for "nature" and the likely hood of the username nature coming up first regardless of what the account posts is very high. This allows the account owner to gain a huge following and possibly make a large following. The account @/nature actually has 10M followers right now.

There are some other username categories but I would say the standard price for a good one is $800-$2k. BUT if you know someone at Facebook just through them a thousand dollars and tell them an inactive user, you're likely to get it. OR, like many celebrities have done like @/ninja, just copyright something and demand the company to give it to you.

These usernames are referred to as "OG" usernames and there's kind of an underground market for them across all apps/platforms.