It's easy to end up with "extras" of the eight basic feet (available from about 1910 on) - this is the Ruffler, binder, narrow hemmer, adjustable hemmer. Early sets also included the Tucker and Seam guide w screw. Later sets included the first four plus the Edge Stitcher and Gather foot. The also came with a large and small screwdrivers, and the bias cutting guide. Over the years there were variations to most of the attachments, if you care to be year specific to your machine.
The others had to be purchased separately, and the vast majority of Singer owners never did, so there are a lot less of them.
Then there are the truly rare - the single and double thread embroidery attachments, and the penguin foot. The full set fagoter is also up there. Most of the specialty attachments run in the $10 - $75 range. The FW crowd helps keep the Singer attachment prices up there, if it fits a FW it often sells for more.