Cheap SSL Certificates are mostly domain validated certificates, where the Certificate Authority verifies the Domain name. CA takes no time (or I can say few minutes) to issue the Certificate. CA verifies domain name via public information of Domain Registrar. CA also asks to verify domain name via either file based verification or email bases verification process.
And Expensive SSL certificates are either Organization Validated or Extended Validated, where the certificate Authority verifies both Domain & Organization. The Certificate issuer needs to go through the toughest validation process. CA verifies in single information or the Organization.
Generally, if you want SSL Certificate for a Single Website with Domain Validated SSL Certificate, you will have to pay the little amount of money, and for Organization or Extended Validation, you will have to pay high.
If you want to add some features like Multi-Domain (security of multiple domains) or Wildcard (security of unlimited sub-domains) the price of SSL certificate may raise.
As the use of SSL Certificates increase since past 3 years, the for price convenience, the users finds Resellers as the best option to buy the SSL. If you compare the price of SSL certificate of Vendor with Resellers, you will find resellers are offering heavy amount of discounts.
Now, If we compare the price of Certificate Authority, then, Comodo, RapidSSL, Thawte, and GeoTrust customers have to pay less but Symantec Customers have to pay high (Symantec adds Vulnerability assessment, malware scanner,