The theory seems to run on a very isolationist notion.From the Discworld series: The Sam Vimes "Boots" Theory of Economic Injustice runs thus:
So, it saves money over three years to have a continual flame spell cast on an object. But you still need to be able to afford three years worth of candles all at once.
Meanwhile, the 50gp cost for a continual flame spell is basically paying for "parts". You can go to the mechanic with the parts to fix your car and it's still going to cost you. The specialist charges for their time.
The Basic Rules put the cost of a 1st level spell at 10 to 50 gp, so a wizard might easily ask 100 for the 2nd-level continual flame. Or 500gp. They can really set their price, since it's unlikely there'll be competition in all but the largest of cities, and it's not like they want to be bothered casting spells freely.
Consider the idea that five families go in together to buy continual lights. They agree to combine their 10gp and pass around the light. With the savings, they buy a second... now savings double then triple etc until everyone has a flame of free light and everybody saves.
As for the wizard, yes, he can gouge the locals in small village USA, but unless he grinds his own meal, bakes his own bread and sees his own clothes... that might not be smart community theory. In fact, in small town USA there is likely not enough gold between them to make it worth gouging them and the ill will.
His smartest move would likely be to do it "at cost" plus discounted goods in future dealings - say 10% off or even just "prioritize my orders" or "save me the juiciest melons" etc etc etc.
It's not like Continual Flame "labor" is this onerous thing. If circumstantially it is bad "right now" telling them to come back next week seems like a good idea.
So, unless you see all wizards as onerous capitalists trying to squeeze blood from turnip farmers, the notion of exorbitant mark-ups seems a bit much as z common thing.
Consider, small villages might have one of each craftsman, likely not several, but that does not equate to everyone trying to bankrupt everyone rlse... the reverse in fact as fair prices and rade for services generosity helped the entire system work.
"If I make you this light, you can work three more hours a day. So I will use your fist and make the light and one hour a day of your work will be making stuff for me at cost from now on,"
Besides, maybe there are clerics of gods of light who might see it as a holy thing to provide these services when time permits.