There's definitely gatekeeping on websites but having done a bunch of crawler work I can say you'd be surprised how rarely a site will outright block you if you

There's definitely gatekeeping on websites but having done a bunch of crawler work I can say you'd be surprised how rarely a site will outright block you if you just do the right things: have an identifiable user agent with a working URL that explains what your crawler does, respect robots.txt, implement polite crawling. As for actually being able to crawl the whole thing though, yeah it's stupidly expensive :-/