Best Digital RCA cable under $100 new or used?

For a little over $100 I found Joe Mazzaglia’s Auricle Audio Design Digital Standard cable to be excellent, and he offers a special intro price that might be closer to what you need.

http://auricleaudiodesign.web.officelive.com/product.aspx

Another excellent RCA digital comes from A'goner Acreyes but I don't know about pricing.

And as others have already indicated, digital IC length makes a big difference to the coherence of the music. 1.5m is optimum.