A tungsten or brass (LAGEOS) sphere covered in optical and radar retroreflectors is cheap and inert. The major cost for that type of mission is the launch.

Given 3-6 months, $<1M all-in, and a team of engineers and scientists, it would be quite possible to get something ready to rock. Such an instrument would allow an ever-improving test of solar-system dynamics for many millenia.

If Elon is reading and plans to do something similar again, get in touch ([email protected]). We'll make science happen.

Or, just launch a dense one of these: http://www.landfallnavigation.com/davis-emergency-radar-refl...