The one you saw in the classifieds possibly belongs to a friend of mine. Depending on just how and what you put together depends on the final cost. When I had mine built I for the most part went with the best I could afford, and got one hell of a rifle, as did everyone else I have read about.
Contact Kirby and he will guide you through the build and you can price things out accordingly. It took me a year of saving to get mine but it has been worth every penny. THere are a few ways to save a few bucks, but after talking about it for half a year and then figureing the differences in cost I bit the bullet and went for the full blown rig.
As for the barel life, it depends on how fast you run them and how well you clean them up. Granted they aren't made for sitting on top of a PD town, but a few of them have. If used for general hunting and casual target shooting it will probably last quite a while with proper care. As for a total number of rounds down a tube I am at a loss to say what that might be.
Here is a post I started a while back to sort of help out folks like yourself as well as others who have these rigs. Sort of a share the info thing. Allen Mag Shooters Group
AS for how do I like mine, well it will reach out and touch something much further, and with more authority than anything else I have ever had the pleasure to sit behind, and do so with hardly any felt recoil. THe hardest thing about it is having to drive up to the country in order to really appreciate how far it will shoot. The ranges here in town just don't do it justice.
There are a ton of calibers out there which will hit fairly high velocities with common bullets, however there are only a handful which will push the envelope. The latter however all come with a price of shorter barrel life. But if your looking for extreme preformance, Kirby is a man with a plan, however he can also work you up a sweet standard hot rod as well.