It seems to vary massively, even within the same corporate agreement. On one that I use, it's impossible to buy HBO fares, so it's often cheaper to go direct to BA.com if you are happy with a "Basic" ticket. On some routes, the corporate discount fare seems to be exactly the same as a plus fare, or at the other end of the spectrum I've saved over £150 on a CE return. I haven't used it for long haul.

I think the short answer to your question is "it depends."