Terrain building on a shoestring is something most wargamers tend to dabble in from time to time, since buying everything ready made will inevitably be rather expensive in the long run. And besides, building your own stuff can be both easy and fun.
Personally, I tend to make most of my terrain multi-scale these days; I want it to work just as well for my 6mm armies as for the 28mm stuff. Take this wheat field/kitchengarden for example. I made it out of cardboard and play sand, plus some spare flocking mats I had lying around.
Next up on the workshop assembly line: homemade rocks!