Here's what I did...
Darning socks is more about honing a skill rather than saving lots of dollars. Although I have found that socks with a higher cotton or wool percentage wear better and darn better than cheap polyester socks. So maybe buying better quality socks and darning them when they finally need it is better than constantly buying cheap non natural ones! I really envy people who can knit socks - I think they would be the ultimate kind of socks to have!
Green-ness: 5/5 for keeping something out of landfill - although a pure wool or cotton sock could go in the compost heap or in the worm farm!
Frugal-ness: 5/5 for using items already in the house!
Time cost: Maybe 10-15 minutes per sock. I managed to do all of mine in an hour and a half.
Skill level: Straight line stiching!
Fun-ness: Great fun to fix something and be able to wear it again!