There are also many different types of certificates.
Basic, Wildcard, extended verification.. etc
The type that you need to buy depends on your use. So do your homework.
The basic cert is $15.00 and will only work for the exact basic domain name.
A wildcard certificate will work for the domain and any sub-domain within that domain.
EV certificates also come in different flavors, and this is the only type that will turn the address bar green.
Be careful that you understand what you are buying based on your needs before you buy.