Which compells a good argument for why magic hasn't yet replaced the common worker in household tasks, however the rules are really geared toward adventurers and small markets rather than mass production of magical items, which games designers really do want strictly limited.
About the only thing I disagree with in your assessment is that washing dishes and sweeping floors doesn't really require much in the way of training; I can't see peasants, indentured servants and other slaves getting paid a whole 3sp a day, but then that doesn't help my case, so forget I mentioned it.
But is it so hard to imagine a noble house in Eberron that uses magic rather than untrustworthy servants? Or a future Eberron where even commoners have access to prestidigitation? Particularly with feats and PrCs that reduce the cost of magical item creation? Not to mention forges that create sentient living constructs, flying ships and magical nuklear weapons?
To examine the at-will prestidigitation item again, at first glance 900gp looks like alot of money because it is. This puts it in the 'noble household convenience item' market well and truely. However, how long is this noble household going to be using this item? 900gp pays for 10 hirelings for 81 days at your estimation, that's approximately a quarter of a year (I'm too lazy to work out exactly what it is). Compare that to a one-time 900gp cost over 1 year or 100 years, or 200, or 500, or 1000! The thing doesn't conk out, it doesn't need room and board, it doesn't need to be fed or have a day off or wages and it never complains and it can be handed down generation by generation for only 900gp! It's an INSANE bargain!
The at-will version of the Prestidigitation item will never be economically feasable, however, particularly for widespread mainstream use because it has no built-in obsolescence. Rather than that, create a 5-use or 10-use rechargeable item and sell it cheap to everybody... but the catch is that you've got to come back for the recharge and you ream them on that. It's called sustainability and it works for cars, household appliances, DVDs, video game consoles, RPG books... practically everything.
Over a period of years, you'll make back the loss you took on the initial sale and then some. People will be able to work longer hours and less manpower will be spent on menial chores, commoners will have more time to enact trade unions and, to make a long story short, you'd eventually get a society much like ours.
I can only see one drawback with flesh-to-stone cosmetic surgury, assuming your DM lets it work. That is, I'd have to submit myself volintarily to a Flesh to Stone spell, and you couldn't make me willingly fail that Fort save. Now, I'm not a noble who probably has about 16 enemies waiting in the wings for a chance to place sharp, pointy, objects in my back, so if i'd feel paranoid, I can pretty much guarentee that a noble won't go for it, particularly when compared to the ease at which I could fund the research of a new spell the formulae to which could rake me in millions of gp.