Even "New England" doesn't mean much, as you could be in some depressed rural backwater or an upscale Boston suburb. In the best case scenario (inexpensive materials and a cheap handyman) I doubt the price would be lower than your $50 suggestion, and the $125 is entirely believable too. The casing and sill for a typical double-hung window could consume somewhere in the neighborhood of 20 linear feet of stock, at a buck-and-change per foot for pine, plus labor. There's also the question of whether the jambs of your windows are flush with the drywall; jamb extensions, if necessary, would require additional material and labor. There's no shortcut to a meaningful number here; you'll have to get some quotes.

Thanks Jon, we have someone coming out for a quote this week, and I wasn't even sure what ballpark he might be in (hence the post). You're right, though, there are too many variables to arrive at even a good ballpark, I guess. We'll see what he quotes us and I'll report back for a reality check.