It sounds like they're talking about when Snickers was first released, which was apparently back in 1930, so 20 cents sounds reasonable. Granted, I'm not sure how much 20 cents back then would be equivalent to now.
Snickers aren't that cheap, but mostly because of the brand name, the Aldi knock-offs that look and taste the same (but have better ingredients) are 59 cents for a pack of 6...
never really understood why snickers are so popular....way too sweet, the caramel is super hard to clean off your teeth and I could buy other chocolate bars for a lower price.