Good will stored are the best place for light weight dress sweaters that are the nice fancy 100% merino wool, casmere, angora, alpaca etc. Not good as a stand alone garment but a fantastic layer and usually so soft you can wear them like underwear right against your skin. I get them for about $7 on average around here.
For a heavy sweater at the best price, mil surp catalogs are about your best bet. I like "Sportsman's Guide" about the best just for variety. If you want a ridiculously thick, warm heavy sweater, click on this link and go to the bottom of the page. Click the "Austrian Heavyweight wool sweater" for $29.97 I bought one and was absolutely amazed at how thick it is. I thought the real expensive ones at cabellas were thick but this one has those beat all to heck and back.
http://shop.sportsmansguide.com/net/browse/military-surplus-sweaters.aspx?d=122&c=2&s=306 One word of caution, I don't know if you are a big guy or not but I'm 6' 2" and about 275#. I usually wear a 2XL tall but can often get by with an XL. I think the one I ordered was the XL but when I got it and put it on, it was so tight I almost needed help getting it back off. Bummer for sure as it was a great sweater. I put it aside for the kids some day. There are other good ones there too. The British military one at top right for $19.97 looks just like the USMC standard issue "woolly pulley" sweater I got when I was in.
If you keep checking Sportsman's Guide and looking at the Military surplus section, you'll find some good deals sooner or later.