Yeah, there's usually a website, or there's a website to tell you when there is going to be one.
Like, for example, here is a site: http://www.animecons.com/events/calendar.shtml/010
Depending on what days you go on or how many days you go for.
Like last year, I went for one day, on a Saturday and I paid on-site. Which was about fifty/sixty dollars, give or take.
(On-site and online paying are different in pricing.)
This year, I'm going to a con for two days, so instead of paying for two separate tickets, I'll get a weekend pass. (One hundred dollars compared to fifty-five.)