I'm researching using this technique to do low-cost terrain mapping for relic hunting.
There is certainly nothing out there cheap enough, but it's looks possible to hack something together.
Here's a thread where UAV builders were talking about this sort of thing just over a year ago. Penetrating vegetation should just be a matter of using an IR laser of the appropriate wavelength - it could even penetrate topsoil/hummus to a small degree.
Thermal might also be an option, though more limited in application. If there's a stone foundation 6" underground in a dirt field, that area would take longer to heat as the sun rises, leaving a detectable area of cooler ground above it. This would require the cooperation of weather, though.
My ideal right now is a balloon-mounted LIDAR-based telemetry package. Attach it to a tethered balloon filled with helium from Walmart, let it do it's thing, then bring it down and hook it up to a laptop. No promises, but I think I could put together such a beast for a retail price of around $20k - not cheap, but not prohibitive, either. About the same price threshold as a GPR.
