Hand-carve into foam... cheap but time (and nerve) consuming 
I did do this for my docks, but that was quite tedious and not recomended for a really large area.
Small areas it is ok, as I just sat in front of the TV and watched that at the same time while doing it.
http://leadadventureforum.com/index.php?topic=30667.30Anything larger than those would have been a hell of a job.
Be warned: Not one for the faint hearted!
(Bottom of that page is also the textured wallpaper board that I ended up settling for. It works almost like cobblestone, but can still be used as a standard rocky plain, I reckon..)