For commercial websites, the cheapest certificates I've seen are PositiveSSL certificates resold by gogetssl [1]. It's $4.55 for one year or $17.25 for 5 years.
Disclaimer: I have no affiliation with either site. I've never used gogetssl, but I will probably give them a shot the next time I need a certificate.
StartSSL has a good deal for commercial websites too - for $120/year you get unlimited certs, including wildcard.
Only downside is they do proper identity checking which is a bit painful to go through (they will need to see official company documents, verify ID by phone etc) - but worth it in the long run IMO.
Yeah and sometimes they just drop the ball on the ID check. I didn't have a bill ready that has my physical address on it (none of my bills print this info) so they said they'd send me a piece of mail with a code in it. Never showed up, all attempts to contact them were ignored. Ended up just getting an $8 cert somewhere.
I will say that once you do get into StartSSL (which I have through a previous company), it's nice to be able to create/sign unlimited certs.
Only downside is they do proper identity checking which is a bit painful to go through (they will need to see official company documents, verify ID by phone etc) - but worth it in the long run IMO.