Best Digital RCA cable under $100 new or used?

Check Cullen Custom Cables. A 1.5m will cost you only $55.
I bought two of these and could not ask for anything to do a better job.

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.

Termination is paramount! More important than wire type, really. 75 ohm Canare connectors are the truest. So, I would go with Signal Cable.

At $99, I found the Transparent Digital Link to be significantly better than other similarly priced RCA SP/DIFs I tried. It was smoother, more musical, less artificial sounding with more natural detail and bloom to the dynamics.

Apogee Wyde Eye. Canare connectors and true 75 Ohm cable. Found them to be very-very good. Could be bought from Sweetwater for below $100.