For the MK25 / S600 combo there are a bunch of different variations that change the price. for the "standard" set the MK25 EVO / S600 is the newest version, and should be priced around $785. you can get special editions of this reg, for example you can get the S620ti which the internals of the second stage are made out of titanium. you can get the MK25 in titanium, you can also get a special edition that is blue.
I have been diving with the MK25/S600 for about a year now and absolutely love it. If you are looking to save a little money, I would also highly recommend either the MK17 / A700 combo or even cheaper yet the MK17 / G260 combo.
The MK25 is a balanced piston design and is one of ScubaPro's top of the line reg's. The MK17 is a diaphragm reg and has been around for quite a while and continues to be one of the best, especially in cold water.
EDIT: My bad, just saw your comment about being a Deep Blue Titanium core. $800 for that setup would not be a bad deal, assuming it is brand new. My guess would be, they have had it in their inventory for a while and just want to unload it.