Radio Shack sells this same cable (save yourself some shipping). My one concern with a variable attenuator is that you run the risk of the dial getting moved accidentally during a recording. A fixed attenuator removes this risk.
The Core Sound fixed attenuator looks like they just soldered a resister inline in the cable. $35 is a lot of money for that. Maybe the best option is to buy the variable attenuator from RS or SP and pick a setting and superglue the control in place. Just a thought.
Good luck.
http://www.soundprofessionals.com/cgi-bin/gold/category.cgi?category=search&item=SP-ATEN-1&type=store
That one's seven bucks. I don't know if it's the same thing, though.