I am not an expert, but I have been buying SSL certs since 1999, so I know a thing or two.
Yes, Verisign's prices have gone up over the years (remember $250 per year, versus the current $850???). The main reason to buy Verisign is the name "Verisign" as the root of your cert. That is all.
You are buying the name. Now sure, you get their cert verification "Secure Seal" icon you can put on your page. Big deal!
But again, it is VERISIGN'S Secure Seal, not BOB's Secure Seal.
A 128-bit cert is the same, whether it be from Verisign, Entrust, or one you issued yourself from your own in-house cert authority server. From a technical standpoint, the certs are the same.
But if you issue one yourself, you will get cert warnings (non-trusted root) until you import the root cert into the IE trusted root CA container. But explaining that to users is a pain. They would rather see Verisign as the cert signer.
Everyone knows Verisign. And they feel safer with them....
And that is why Verisign is so expensive!