Cyclocross - How much costlier is it to build a bike with components?

(please excuse the silly title.. obviously a bike without components would be cheaper, but more difficult to ride)

I'm attending a charity auction on Saturday, and I'm told some frames and forks will be up for grabs. While I have been shopping complete cyclocross bikes for awhile, this seems like a good opportunity to get some choice gear -- and to end up with a unique ride. Does anyone have a ballpark estimate as to how much more expensive it is to build a bike compared to buying its equivalent? I've been looking at bikes with entry-to-moderate pricing (e.g., Jake the Snake).

If you just go to a decent online bikestore they will have complete groupsets for sale. Just choose the one you want, and the wheels, and you'll know roughly what the cost will be. Expect it to be higher than you'd like though - it's not unknown for people to buy a bike from Bikes Direct just as a cheap way of getting of getting groupo (they then ebay the frame.)

Of course, used parts off ebay can work out a lot cheaper. And so can new old stock for 8 speed, and you can save an awful lot of money if you're willing to use bar-end shifters. Really, you're the only person who can work out the price because you're the only person who knows what parts you'll settle for.

Another option could be to buy a used non-crosser, transfer the parts, and then sale the frame. The best deal of all would be to buy a used non-crosser in good condition, but in most places they're rare birds. Oh - and you could think about buying a bike from Bikes Direct: they have Sora-powered 7005 alu crossers for less than $500 and quite a few people like various BD models. If you're up to building a bike you'd certainly be able to do the tweaking that they sometimes require:

http://www.bikesdirect.com/products/cross_bikes.htm

It depends on how hard you're willing to work at getting the components, where you're willing to buy them from, whether or not you're willing to buy used and what you have already in your garage. If you spent a lot of time on eBay and various UK web sites, you can shave the difference down a bit. I've also found that asking on the local racing e-mail list can yield some good buys, as those guys are constantly upgrading their bikes and tend to have a lot of stuff they pulled off of new bikes (replacing Ultegra components with Dura-Ace, for instance).

To give you an idea, here's a quick sample build I threw together from web sources.

FSA Gossamer crankset: $50 (eBay)
105 STI levers: $155 (Ribble)
105 Cassette: $42 (Ribble)
105 rear derailleur: $38 (Ribble)
105 front derailleur: $26 (Ribble)
SRAM 10-speed chain: $24 (Ribble)
Cane Creek S3 headset: $30 (Universal)
Tektro CR720 brakes: $38 (Tree Fort)
OpenPro/Ultegra wheels: $300 (Bicycle Wheel Warehouse)
Michelin Mud2 tires: $68 (Tree Fort)
tubes: $8
Salsa Moto Ace Bell Lap Bars: $34 (Universal)
Profile Design Aris stem: $40 (Universal)
Kalloy Guizzo seatpost: $37 (Universal)
Crank Brothers Eggbeater pedals: $45 (Universal)
saddle: $30
bar tape: $15
cables: $20

So that's over $1000 plus shipping and you haven't even got a frame yet.

I've found that it is generally less expensive to build the bike the way you want it versus buying a complete and having to change things to get it the way you like it. An example would be my current CX bike. The build came in at just under $100 under what a brand new bike would have cost me, but my bike has better frame/fork and components across the board than a production bike.

Thanks, folks! I was thinking about making a two-speed (36-46 or some such with a tensioner), which might save me some dollars in the components department.

Anyway, I'll keep all of this in mind when I decide if and how much to bid.