i picked up a BK scope on ebay a month ago. just set up your search to exclude anything above 20mhz, tektronix and HP scopes and you will be able to find plenty for $20 or less.
Nothing wrong with HP and Tektronix, they are just such well regarded name brands that even a gutted 20mhz Tektronix fetches a high price.
And I do just FINE with my single trace scope! (oh wait it is dual trace, there is another input on back)