A blued Single Ten is a distributor exclusive through Lipsey's and, as such, you'll most likely have to have your local Ruger dealer order it for you. Odds of walking in and finding one in the display case are minimal. Usually not a big deal to order one if the distributor has one in stock, so if you want a blued ST, just have your LGS order it for you.
For the fixed sight, tapered barrel MKIII stainless, that's actually a typical price. Have seen them at our LGSs in that price range. It's when you go to the bull barrel and adjustable sights that you start to pay a bit more.
The problem with the tapered barrel models is that darn front sight which is too low in height when you replace the fixed dovetail sight with an adjustable sight. The good news is that the fixed sight models with tapered barrels are usually right on, out of the box. Most importantly, these "fixed" sights are easily adjustable for windage by tapping the rear sight in the dovetail as needed.
Elevation is more of an issue. If your gun is shooting too low, you can file down the front sight. Too high and you can actually file down the top of the rear sight. These are extreme remedies and not for the faint of heart and not something I'd recommend to anyone who hasn't done this, before. it's slow and painstaking work, file a bit, shoot, file a bit and shoot. Get even a little bit too happy with the file and there is no return button to hit.
Again, I've never needed to do either a windage or elevation adjustment on a fixed sight, tapered barrel Mark.
One last option is replacing the rear dovetail sight with a Volquartsen Dovetail Scope
mount and then adding a red dot, such as a Bushnell TRS-25 or other red dot that uses one bolt for mounting. This would be the easiest and quickest alternative to messing with trying to adjust those fixed sights.
Best of luck. I own Marks and a Single Ten and they're both terrific guns.