This post is very timely. I have just started looking at Open myself. I have done a lot of research and the gun is just the begining. Don't forget about the $500 for five tuned mags, then the brass, bullets, holster, powder, and new re-loader set up. With a $4K gun (http://www.predatortactical.com/High-Capacity-Pistols/-Banshee-Open-Gun-p300.html) I am looking at a total investmetn of $6K just to fire the first round.
If you are serious about building an Open XD then I would recomend you send your gun to Powder River Precision (http://www.powderriverprecision.com). Dan is a good guy and has a lot of experiance with XD guns.