Also keep in mind that even if the $420 fare appears on the discount sites, there's no guarantee that it's actually bookable at that price. There's a decent change it will say "sold out!", or "oops, your fare has increased!", or "sorry, we can't confirm it with the airline" after you try to buy it.

I've given up on even looking at them now after this happening 3/3 times I tried in the past.