They are typically used by women who have had at least one child, because they are usually more difficult to insert in women who haven't had children. Also, they don't protect against STDs, so you have to use a condom, too. You don't need a prescription. You can just go in and buy them. For all others, you have to see a doctor or go to a family planning clinic, like Planned Parenthood.
Clinics are good choices for teens.
They usually only charge what people can afford to pay. These are basically high doses of birth control pills, taken within 72 hours of unprotected intercourse. This should not be used as a regular method of contraception. Like the name says, it's for emergencies only. The cost of contraception varies, depending on type and where you go to get it.