I find installation costs to always be significant. That is why I am not really a fan of using used equipment for new installations. If you're doing a slide-in replacement, then you can have significant savings. However, for a new installation of an audio panel, navigation system or engine analyzer, the installation cost can be close to the cost of the new equipment. Thus, a 50% saving buying a used device is only a 25% saving for the overall installation, and at that point, having the latest equipment can actually save cost in the long run because the equipment is less likely to become obsolete or break from old age.
I think replacing the antenna cables while you have the plane open can be a good idea. I installed an INS429 VOR in an older plane, and was having issues with reception of the VOR signal. The connection at the antenna was corroded and the cable had degraded over time. It wasn't all that expensive to pull a new cable and have a known good connection while the panel was open.
