Ed: My bet is that it's mostly about tooling and set-up costs. Carbon steel production runs would have to be machined on a seperate line, and as a seperate production process from SS units.
Just as one example, the carbon steel barrels would require different 'speeds and feeds' from the SS for rifiling, as well as broaching operations etc.
Any mfgr is going to look at costs from a 'per unit' standpoint but with the goal of producing 100k or 200k unit lots that can actually be sold. The marketing guys know how much something might cost to produce by the unit, when 200,000 units are produced. They will calculate a wholesale price based on the amount of pre tax profit they need to generate.
When it comes to introducing a new product line, say a Blued SP 101 .357 in this case, most mfgrs have sales 'scouts' (for lack of a better term) who canvass wholesalers and even retailers about how many units they might consider buying at $xx. If the demand looks luke-warm, then the actual no kidding sales will likely be lower. Translation: if the pre-production demand weak, then there's a small chance a company will invest tons of resources into a questionable line. The Edsel was a classic case of a company not doing their homework first; Ford lost mucho bucks on that goose-egg.
Also, remember that Ruger primarily uses investment castings. SS castings are going to be produced using diferent methods than carbon steel would be. In fact, carbon steel castings might not pass all the engineered requirements for stress and fracture resistance in a firearm. So Ruger would be back to machining a carbon steel billet; not likely since that's not what they do primarily.
Sorry for the long post, but I thought it might help answer your question better. Botttom line: It's all about the BOTTOM LINE!
John