The value of the certificate comes mostly from the trust of the internet users in the issuer of the certificate. To that end, Verisign is tough to beat. A certificate says to the client that you are who you say you are, and the issuer has verified that to be true.
You can get a free SSL certificate signed, for example, by StartSSL. This is an improvement on self-signed certificates, because your end-users would stop getting warning pop-ups informing them of a suspicious certificate on your end. However, the browser bar is not going to turn green when communicating with your site over https, so this solution is not ideal.
The cheapest SSL certificate that turns the bar green will cost you a few hundred dollars, and you would need to go through a process of proving the identity of your company to the issuer of the certificate by submitting relevant documents.