Probably around $50 or so. When I built mine, including all of the tubing to connect to the keg and CO2, the cost wasn't that much different from the one that northern brewer has, including the tubing (items #7206 & 7207), but I had to get 3 ft. of the stainless from McMaster-Carr. Solbergg has PM'd me about it, so he's got first dibs. I'll price all of the brass fittings at home depot, and I'll make it with whatever connection type you want for gas and liquid.
If you look at the Northern Brewer one, they just have barbs on the thing, which you attach the lines and have to clamp them on. The other ends of the tubing are the flare fittings to attach to a liquid out on a keg and to your CO2 system. Mine does not have barbs. I use a flared fitting on the liquid in side which then has 6 ft of tubing connected to a liquid out ball lock. The gas side is very different. All of my CO2 lines are 1/4'' push connect fittings so I can completely disconnect any lines not in use, or just plug into the manifold to get CO2 anytime.
I will build it however you want. With the tubing, or without, in whatever connection configuration works on your system. If you want to do the tubing yourself, you should be able to find the parts at your LHBS or any online homebrew supply.