The prices I changed it too are staying.
Are you sure? Because the $20 cards are still cheaper than the $50 cards at the prices you've listed now...
I mean, not saying you need to change them or anything, it's just that generally it'd be the other way around
Math:
Let's say you wanted to buy $100 worth of cards:
$20 Cards: ($15) * 5 = $75
$50 Cards: ($40) * 2 = $80.
Alternatively, you could consider that the $20 Card costs $0.75 (15/20) per dollar on the card, while the $50 Card is valued at $0.80 per dollar on the card.