I recently discovered that there's a small movement of internet fibre geeks dedicated to taking thrift store sweaters apart for yarn. Seems an obvious thrifty choice when you think about it - after all, if you can make one, you're more than qualified to take one apart and reuse the yarn for your own projects (especially when you're relieving the world of incredible ugliness). My local Goodwill sells sweaters for an average of $5-6, when the same amount of decent quality materials would cost at least four times that.
Here's the first sweater I thought I'd try - I paid $4.99, and it had all the right qualities for the first attempt. Worsted weight yarn, big stitches, neither loose or tight, a good mix of wool and cotton.
Via the Ravelry Thrifty Knitters group (registration required) I used this tutorial for help and got to work. 12 hours or so later, after a slightly steep learning curve in spotting the right stitches, here's the finished product:
I didn't measure the yardage (I still haven't built the yard plank I want), but it weighed around 560g - more than enough for a project in the future. While it took a lot of time (an entire day! My arms hurt afterwards!) I expect that experience to vary in the future as it gets easier to spot the worthwhile projects on the rack.