If you want to pay for one it costs about ten bucks a year: https://www.gandi.net/ssl/grid
If instead you want a free cert: http://www.startssl.com/
The more expensive certificates offer insurance, but if you read the small print you'll see that it isn't worth much.
Also worth noticing is that SSL comes free for one year with domains bought at Gandi.
Pretty much every large company uses VeriSign, which isn't cheap. Twitter, Apple and PayPal all use EV certificates (which make the url bar go green) from VeriSign which are between $1000 to $1500 each.
It kind of comes down to whether you want an EV cert or not. Certain sites are cheap for non-EV certs (Like Comodo), and others offer cheap EV certs (GeoTrust and DigiCert will sell you one for $300/year). The cheapest for everything seems to be GoDaddy, but, I guess caveat emptor.
SSLs.com (formerly cheapssls.com) has them for reasonable prices like this one you can get for 5 bucks a year (which is ultimately issued by Comodo).
http://www.ssls.com/comodo-ssl-certificates/positivessl.html
Google Comodo SSL fraud for a better picture.