Hi Macrojack,
The amp was released in the late 70's/early 80's and is still the same basic amp. I can't tell you what changes were made to the design since Richard isn't telling. However, they are always under constant development. So R&D cost would certainly be part of it. Also, these amps are still hand made and individually tuned by Richard personally before shipment. That level of personal involvement simply costs more today.
Sorry if this sounds like tap dancing, but it's all I can tell you. Even when I've had mine upgraded he won't tell me what he changed. Even opening it up (I'm not an engineer...) it looks basically the same. They sound a LOT better though!
I know it's not the answer you were looking for. If you're interested in the amp though I'd say that you should compare it as a current version to what's available today. You'll be shocked at how good it sounds. Then, when you compare it to other amps at the different price ranges I think you'll find that some may be the same quality of sound, but they'll be different (and typically MUCH more expensive).
The one thing to remember with the BELs is that they aren't romantic, yin/yang amps... They are really a very neutral amp, so what you put in, you get out. I know that sounds like hyperbole, but I've had these same amps in other systems and it's just proven to be true. All the other amps I've listened to have had more of a sonic signature.
Last, you should consider getting Symposium Ultra shelves for them as this is what Richard tunes them on. They should therefore be considered part of the amp "system".
Cheers!