Hobby Lobby, like was said above, has a sale on yarn almost weekly.
I never buy full price yarn. Have not for years.
Hobby Lobby has a yarn called Icelandic Jewels, regularly about 6 dollars a skein, but, recently, they were all on sale for ONE dollar each!
They were out of a color I wanted, to complete a project, so, I got a rain check, and when the new shipment came in, I got 7 skeins of yarn for 7 dollars, not 42 dollars.
Just figure that most projects will take about 7 skeins of 3 oz. yarn, so, stock up when the sales are on.
Ebay sellers sometimes get greedy, and charge too much for too little, so, I steer clear of them.
JoAnn Fabrics has a sale on right now, here, at least, 50% off any regular priced item. Plus, their yarns regularly go on sale. I know, because I work there.
Seasonal yarns, such as pastels for spring, christmas yarns, fourth of July r,w,and b yarns will drop in price after the holiday, so, keep an eye out, and stock up.