Have had a few requests from environmentally sensitive clients to use one of the porous pavements, including two this year. When told the cost is twice asphalt or concrete, they aren't quite that much of an environmentalist.......
I have seen a few architects try to use gravel. If you use it on slopes, you really, really, need to control drainage around the paths on slopes and low points with basins, etc. Drainage alone probably makes the cost close to asphalt, although, truth be told, you should have a lot of drainage around concrete paths as well. On clay soils, not sure packed gravel drains enough water through, and it starts running on the clay base, or the surface, leading to wash outs.
However, on the landscape architecture side of things, porous pavement is becoming all the rage as people focus on reducing run off in urban situations.