Numrich seemed to have the best price on factory barrels, but they have the chamber indicator notch (if that matters to you). New "regular" factory 9mm barrels seem to go for $175+, and the Numrich deal was too good to pass up for my purposes (range use).
For the peanut gallery:
$326 for the slide conversion "kit" does seem kind of high, but it is still way cheaper than another complete P226 (unless you go for one of the used CDNN "good-to-fair condition" lottery guns which seem to be selling for ~$350-$400).
In comparison, the shipped price of a conversion barrel "kit":
Barsto or EFK conversion barrel = ~$200 (+plus three month wait time)
Gunsmithing fee for fitting the conversion barrel = ? (BarSto will fit for a small fee, but again with a wait time)
recoil spring = ~$10
9mm magazines (e.g., 3 used) = $65
~$275+, but that price is with your slide's sights that may be off. If you want to shoot casual IDPA, then you would have a problem.
Incidentally, my "CDNN special" slide came in last night, and it had the SIG I-dot factory sights. All the other parts came in, too, and everything fit together on the frame and passed a function check. I will get it to the range when I get a chance, but probably not for a week or so. . .
I took a look at SIGForum last night, and it seems that you can go up from 9mm to .40/357 with a slide swap, but the cost may be steep due to the high dollar price tag on the stainless slides used for the higher calibers (compared to the older folded/stamped metal slides for the 9mm like what I got from CDNN).