Why not a MER?
Take a MER rover, attach a wench with a few thousand feet of fishing line, and have it carry a probe fitted with a light source, camera, and a couple of other instruments. The rover would approach to within a few hundred feet of the hole and fire the probe via a CO2 cartridge. Afterwards the probe could be reeled back in by the rover.
Such a mission could be performed with minimal cost as the MER design has already been developed and tested.
