Green Man Gaming has Premium for $49.99, but they always have a 20% coupon code for ANY PC downloadable games, including DLC.
20% of $50 (rounded on Green Man Gaming) is $10.
$50 - $10 = $40.00
20% coupon code = GMG20-FXYD1-7WHP1
If you're lucky, they sometimes put up a 25% discount code which would reduce the price even lower to $37.50.