what kind of servers warez forums use(users amount)

[rhemux]

1-warez foruns with a userbase of 10k or 100k , browsing and posting links and stuff
what kind of server and cost, are we talking about

[FransTormer]

You for real? I reckon you're talking about a webserver here serving the entire site? It all depends on the processing-time for each request which is quite logically linked to any images, links, avatars and so on and so on.

No set specs for server, you just need scalable software which you can easily migrate to a bigger server or load-balance over several servers.