It depends what style and how much they want to charge really but check around and see what style you like best they normaly let you try it out and see if its what you want and it is lots of fun.
Different places have vastly different prices, the only way to find out is to go to your local school and ask. Don't just go to the first place you see though, shop around and sit in on classes.
Like everyone else says... it just depends on the sensei/school. For instance my sensei doesn't charge anything. The classes are free.... but they do charge for belt testings and tournaments.
The payment depends on the number of lessons you want to take. For me its 20 for a month with one lesson a week. If you pay 30 a month you can do as many lessons as you want for that month. Or that is for my style lol i hope you enjoy it karate has a rich history behind it so your interest in it should grow