933 uppers are probably some of the hardest to find, and the most coveted. A Colt collector would probably pay close to a thousand for the upper alone, if not more.
Wish I could make you an offer, as the 11.5 lightweight is one of the coolest uppers out there IMO, but I'm one of those champagne tastes on a beer budget kind of guys.
You can't compare the price to what people pay for a 6920. 6920 are fairly easy to find, the 933, not so much. It also wasn't that long ago that retailers were asking $2400 or more for a 6920, they could get away with it because they were hard to come by.
Some 6520 uppers were in the EE not too long ago. Asking price was $900 for 16" carbine gas, lightweigt profile, A2 carry handle upper. Not a very desirable unit to most, especially at the price point, but I do believe that they all sold because collectors want them and they don't pop up too often.