The Hot Toys and Sideshow's own sixth scales at list price tend to be cheaper than the premium formats or other statues, It's probably a case of the aftermarket raising prices after the figure went out of production. It is worth noting though that unlike a lot of other toys, it can be kind of a crap shoot as to what retains or gains value. My Hot Toys Marty Mcfly was bought off ebay a good while after production ended and from what I remember was right around the original MSRP, I've also gotten some older Sideshows from when they were just starting out for dirt cheap. So my guess is you just ended up grabbing one that actually has grown in value since it's original release, while your premium format is still being actively produced (to my knowledge). There are some older premium formats that have gone up into the thousands once Sideshow stopped selling them.
I'm digging the look of the Keaton figure, they really seemed to go all in on the rubber look of the costume, I'm also assuming the edges of the cape have some kind of bendable material for posing?