>Why are their costs so high if they can just host magnet links and cache the site with Varnish or something similar?
Just from personal experience, but here is my view of it.
A good search engine for millions of torrents costs several hundred a month. Scrapers to keep torrent seeders / leechers up to date cost another fifty to hundred (depending on how up to date you want your torrents). Now add in a few website servers and a load balancer for another hundred or two. You are looking at three to five hundred a month. Doesn't seem like a lot, but this is just a smallish torrent site. Isn't large enough to generate ad revenue or have enough donators to keep running. Maintaining that by yourself soon get's expensive unless you have a good job with plenty of disposable income.
Just from personal experience, but here is my view of it.
A good search engine for millions of torrents costs several hundred a month. Scrapers to keep torrent seeders / leechers up to date cost another fifty to hundred (depending on how up to date you want your torrents). Now add in a few website servers and a load balancer for another hundred or two. You are looking at three to five hundred a month. Doesn't seem like a lot, but this is just a smallish torrent site. Isn't large enough to generate ad revenue or have enough donators to keep running. Maintaining that by yourself soon get's expensive unless you have a good job with plenty of disposable income.