Turning Archive 2004
Check the prices in the turning catalogs. Also go to eBay and check the "completed items" to see what sold and how much. Checking the current auctions only tells you what people are "asking".
As to the size, I would buy any size blank knowing I could cut it up for bottle stoppers, ornaments or pens. For instance, I have a 12" swing but could get a 10" bowl plus a box or ? out of a 20" blank.