It costs more so obviously if you throw them on you have to charge more for the deck.
That's why I am partial to the Kickstarter system wherein the price for the Vortex is set in stone, and if we make profits, those profits go to adding little things like metallic inks which make the deck look better. Even if it's 1% better for you, your price stays the same.
What I think is hilarious is when "metallic ink" is hyped as a core feature. Look at the Artifice, and tell me when did you see their marketing department smear "metallic" all over your faces? I was personally surprised to see my Artifice have silver metallic ink. It's a subtle yet cool change.
Metallic ink can be a huge downfall (Gargoyles) or it can be absolutely sweet.
tl;dr:
Don't use metallic inks in your decision making process of whether or not to buy a deck. If you decide to buy the cards, enjoy the metallic ink as an added bonus to make you a tiny bit happier.