The hardest to find Singer attachments seem to be
1. Penguin walking foot
2. Style-o-Matic foot
3. Double Thread embroidery attachment
4. Single Thread Embroidery attachment
5. Fagoter (full three piece set)
The Fashion Aid case is also highly sought after.
For the most part, the rest can be picked up for under $100 (unless you want to pay more, there are always sellers asking for the moon).