I ran into a similar problem a few years ago and had to pay about $80 for a
new one from a marine elex repair shop..

However, if you got the patients, you can monitor the eBay listing since
there are cables showing up there frequently.. Mostly, what i have seen were
for Furuno but I sometimes see Raytheon cables. If your model is fairly
common and been in service for more than a few years, the chances of getting
a used cable cheap are improved.. The used cables I have seen are generally
going for about $25.

Good Luck..

Steve
s/v Good Intentions