Supply and demand comes into play but one has to consider that even if millions were made, how many are actually available on the open market? Rifles owned by people unwilling to sell them cannot be considered into the supply side of things since they are not being supplied to the market. If there were stacks of crates of one type of rifle sitting on warehouses and stores, available to be sold, prices would reflect that. A current example is the Mosin Nagant. When there were millions ready to be sold, they where going for $50-100. Now they're getting sold and ending up locked up as private citizens property and no longer available for the supply side of the market so prices are going up.
The Lee Enfield rifles had exactly the same thing happen to them. Their numbers have been depleted somewhat through accidents, lawful destruction when they're used in a crime, unlawful destruction by some governments, or just getting worn out and tossed as scrap metal. Even with, at minimum, a couple hundred thousand in Canada, the overwhelming majority of those cannot be looked at as part of the supply side of the equation as they are not available to be purchased.
Garands cost $1000-1400 on average but what if tens of thousands of them entered the market as items for sale? Say everyone who owns one decided to sell it, all at the same time, or the Korean Garands got imported into Canada (since we're talking hypothetically and not in reality). The price would plummet as the demand is now dwarfed by the supply.
The price of SVT-40's went up $100 when the Ukraine-Russia conflict started. Turned out a lot of the surplus SVT-40's that ended up in Canada were coming from Ukraine and couldn't as easily be exported anymore. The local supply (to Canada) was diminished even though the actual number of goods in existence didn't change. Thus the price went up.
The same SVT-40 that sells in Canada for $350-450 is $1000-1500 in the US. Why? Practically no supply, but a decent amount of demand.
It's all just basic economics.