Piero, the ideal "basket case" is simply a more or less complete bike that somebody else took apart and then gave up on for one reason or another and now simply wants to get rid of. In American grease monkey mythology, basket cases are usually sold for a song and contain almost everything you need to build the bike of your dreams. Unfortunately, basket cases are a little bit like "barn finds" these days. They are rare and when they do turn up, they are probably expensive.
Joshua, there was a barn find sold at an H&H auction in the UK last year (2018). It ended up going for just under $9000(!). Admittedly, they are worth more in the UK than they are in the US.