Inspired by Anthropologie's Silken Jabot Pullover:
My version cost under $5 - thrift store pullover + thrift store "silk" fabric
And here's a quick how-to:
1. Cut a long length of silk/silk-esque fabric, 6 1/2 inches wide. You want it long enough that it will go around the neckline and still have plenty to tie. It's better to cut it longer than necessary and then adjust later.
2. Sew right sides together lengthwise.
Then turn, so your seam is now to the inside.
3. Find the midpoint of your strip of fabric and start pinning it to the back, inner, mid-point of the pullover.
Pin around the neckline to approx. 1 inch of front midpoint.
4. Stitch in place.
5. Tie and adjust length of ties (mine were a bit too long). Flip raw edge of fabric to the inside, pin and stitch down close to the edge.
6. Wear and enjoy, especially knowing how much money you just saved yourself!