system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why does the mint sell "$20" silver coins for such large sums?

Why bother putting a price on the coin at all? It's not like a $20 coin is even a thing out of this context and they always cost over $50.
gpt
Non-circulating legal tender (NCLT) coins are never intended to circulate as cash money but they include a denomination as a fail-safe of sorts so that should the world economy collapse and silver becomes worth less than $5.00 per ounce (for example) then at least this item's value is backed by a treasury for $20.00. 

Circulating coins used to be made of actual silver and gold but these were phased out because the value of the silver and gold outpaced their face value and it was easier to make more durable coins out of copper-nickel and steel cores. Canadian "junk silver" coins up to 1966/67/68 (depending on the denomination) are all currently worth 7 to 11 times their face value. 

In reality, most of the NCLT $20 silver coins don't even include an ounce of silver and so they are essentially jewellery disks. If you like the art/design then buy it but don't expect to hand it off to your great grandkids to make a fortune. They won't. It is a crappy investment in almost all cases.