I buy ours at Walmart. Last fall I got 2 of the 4GB for under $10 for the pair. They were perfect for schoolwork. I have never priced out the higher ones, and I don't know that there is a quality issue.
On black friday, they typically are almost giving them away. That's when I usually buy one or two. I've had a couple of different brands, and not noticed a difference between them.
I'd also check walmart or see if Best Buy/Staples/Office Depot are having a sale on any this week.
My son was required to have one for school last year and we got it at walmart when school supplies where everywhere and only paid $6. It worked great and survived many trips in the washer and dryer, but he ended up losing it. I was able to get a new one at Walmart and paid around $9. I do not recall the brand.