The last time I saw a penquin foot selling on EBay (a couple of days ago), the last bid I saw was $249 USD.
Why would they be worth more than Swiss zig-zaggers? I use my Swiss ZZ as a walking foot when needed. Just set stitch width to nothing and it's just a walking foot. Did this on a 201 for making upholstery vinyl piping before I got the roller foot for my 96K.