The answer: too much. It was 2007 and everything was looking up, up, up. Oops.
I bought the diamond from a broker who had the specific cut from the specific cutter that I wanted. Then I took it to a few different jewelers to look at setting options.
I would not buy a Tiffany engagement ring. Honestly, beyond the huge premium (100% more than what you'd pay for the same stone yourself), the workmanship on their mass-produced rings is pretty awful. The pave is a joke. Unless you are buying one of their one-off or special collection pieces, you're getting a mold-made setting off an assembly line. Sucks. It's the equivalent of a fused, RTW Armani Collezioni suit. Cartier and Harry Winston do a much better job. At least their pave settings are properly hand-wrought--but they will both admit, with some prodding, that they don't do the work themselves. Guess who does? Some dudes in the Diamond District. I looked into some Europe-based jewelers as well. Apparently, the best pave work is done in New York City these days. That's where everyone commissions their work. If you can do some moderate sleuthing, you can figure out where to go, save tons of money, and get your girl a better diamond in a better ring.
If you are going to spend big money on a stone, find a skilled jeweler who does truly bespoke, hand-wrought work to do it justice. It is potentially much more delicate and refined looking than mold-made work. But here's the key: make sure whatever method the jeweler uses, he is making a custom, one-off setting sized to your diamond(s). You don't want a mass-produced piece that's just had its prongs adjusted. A true custom setting must be made--you can't just pick it out from the display case. This particularly important if you are doing pave. Pave in pre-made settings with uniformly-sized spaces looks terrible since each pave stone is a slightly different size.
My biggest mistake was insisting on a D colorless diamond with VVS2+ clarity. I paid a stupidly huge premium for that. I could have easily gone with a G-color stone and VS2 that would look just as nice but been either half the price or 50% bigger.