Fingerprint.com charges around $0.004 per investigation, and really encourages you to set up a first party proxy - so iOS doesn't nuke storage after 7 days. And even there, unless its a cookie match; your ID usually carries a confidence number.
That should give some hint about how difficult it is to do in real life, at scale.
That should give some hint about how difficult it is to do in real life, at scale.