Of course, that is a somewhat reasonable option. But I actually have a D7000 (not D7100), which I have read to understand may have a better sensor for this hobby. Not to mention that I use the camera for a lot more than this... Astro is a very minor aspect of what I use my camera for.
If changing cameras, I think I might just go mirrorless. But as it's an evolving tecnology, I think I'll wait for another generation.
Ultimately, I just feel $150 for a cable is nuts. It's less likely to be a recoverable amount if sold, whereas filters, etc. would easily be....... I can't blame Nikon. It's not likely they could foresee this particular use for this particular model, and didn't engineer the functionality into it until a year or 2 later.
I'll make my own. Should be a piece of cake as I have access to this type of tech through work. If it triumphs, or fails, I'll post about it.... This could take a bit of time as ordering stuff for Canada takes ridiculous time for some reason.
I do appreciate the comments and suggestions, regardless of whether we agree on certain values. It's all a learning curve!