What's the cheapest Steam badge?

Hi all,

as this "summer adventure" activity requires us to craft badges in order to participate, I kinda started wondering which bdages are particularly cheap to craft? Fact is all badges cost you money to craft, whether it is because you buy the cards or opportunity cost because you didn't sell them off the market. However, not all badges have the same cost to craft. The sets with the cheapest individual cards aren't necessarily the ones that are chepaest to craft, because they may also require more cards. I'm not asking whether it's sane to craft badges at all just to contribute to your color team or get a summer sale card (it probably isn't) - I just wonder which badges are particulary cheap to craft. So come up with suggestions...