NOTE: YOU HAVE TO LINK YOUR BNET ACCOUNT TO YOUR TWITCH FIRST
So from the screenie above and other people it appears to be 3 credits per match watched.
That means you have to watch 34 matches for a skin.
I've done the math, if you watched all 34 matches on a laptop in New York the electricity would cost you about $1.50 (much lower pretty much anywhere else, NY electricity is pretty pricy).
So it's really more worth it to watch the matches than to buy the skins
P.s. each match is assumed to be 2 hours long
and if you use a desktop it's around $2.10 for a skin in New York
Sources: http://smallbusiness.chron.com/laptop-vs-pc-power-consumption-79347.html
https://en.wikipedia.org/wiki/Electricity_pricing
New update: https://www.reddit.com/r/Overwatch/comments/7t43b6/watching_owl_with_your_bnet_account_linked_to/dtaa5zm/
Tl;dr we all got baited