It would definitely depend a LOT on the quality of the shells- I would definitely pay much more for a Vaughncraft than a Keller. In fact, right now I'm planning a custom kit project, thinking of ordering shells and building it.
A set of shells for a standard 5-piece kit done by Vaughncraft would cost around US$2300 for the shells alone. Then again, Vaughncraft is possibly the top producer, or at least among the best, in the world; even big companies like Pearl or Black Swamp percussion buy Vaughncraft steam-bent shells.
Hardware is surprisingly expensive, too. A lug from one of the big companies can easily cost over S$30. Pearl higher-end lugs, such as on the Masters series, are around S$40+, while Sweelee quoted me S$66 per lug for the Warlord series snare drums (yes, this means that during the Sweelee Half Price sale, it's cheaper to get a Warlord series snare than to buy the lugs for the snare =O) On the other hand, cheap made-in-china lugs can be obtained for less than S$10 each. Somewhere in the middle, you can get custom-made lugs for between S$10 to S$20. Of course, if you're getting custom, you'll probaby want top quality. You'll also have to look at the different types of lugs- most of the custom made ones are insanely lightweight, which is generally good all-round. So you'll be looking at another S$100+ per drum, for around S$600.
Then, there's hoops and mounting hardware. You can either go flanged steel, or die-cast or wood. A pair of flanged hoops would cost between US$30+ to US$50+ for each drum, depending on how big the drum is, while die-cast or wood hoops cost 2x to 3x that amount. So you'll be looking at a minimum of US$200 altogether for the hoops. Mounting hardware, well, since we're going high-end again, we'll probably be looking at RIMs mounting, which would set you back around US$300+ altogether. Adding in tension rods, bass spurs, floor tom legs... this would be around US$550 for the drum hardware.
So far it's gone up to around US$3000+, almost S$5000, for the raw material cost alone. Now, let's start talking about the cost of workmanship (trust me, dye-ing, staining, lacquering and all is a horribly tiring and time-consuming job and deserves to be charged a premium- plus, most custom-builders would want to cover the capital they spent getting the specialized equipment they bought to help them do this)
Add in the hardware, like snare stands and cymbal stands and pedals and all, and you can probably be looking at S$8000+++ easily for a high-end custom 5-piece kit. And if you compare this to the high-end kits from big companies...
On the other hand, if you simply get Keller shells, you'll be looking at much cheaper options. It all depends on what kind of stuff you get. I'm also set on building a small 4-piece acrylic kit in 10x7, 14x14 and 18x14 sizes (and using my existing snare); the shells will set me back only about US$500 (as compared to in the thousands) and since there's less drums, there'll be less hardware costs too...
What interests me right now, though, is how much someone would be willing to pay ME to build a custom kit for him (or her) =P
Edit: By the way, the Vaughncraft-shelled kit that I'm planning to build is in 10x8, 12x9, 14x13, 16x15 and 20x16 configurations. I will be using my own snares, hardware and so on, so I'm just getting a shell-pack and rims mounting. That alone will set me back around S$6000 or so (and I'm doing the dye-ing and lacquering myself) and so I'm frantically trying to save up the cash to get it.
but that is another topic for another day 
