I bought a ferguson 2-bottom (14") plow on Craigslist for $250 -- very good condition with coulters, joiners and tailwheel all in tact. Have seen a few since in the same price range. New paint applied by me.
The discs on a two bottom plow (and in the photo) are the coulters, which cut through the sod and help prevent field trash from catching on the tops of the shares/moldboards. They are not always necessary elements for plowing.