My first terrain attempts all had warping issues, then I found expanded PVC. Expanded PVC is plastic sheet with very small air bubbles in it. It gives it characteristics very similar to wood. So you can cut and sand it just like wood and it is totally resistant to water or warping.
I've used 3 sizes of expanded PVC for basing. I did a project with a bunch of hirst arts floor tiles and needed a very rigid base so I used the .24". To use it you need power tools to cut it, I tried it by hand and the results were not pretty. But by roughing them out on a table saw, I could easily sand down any mismatches.
The .08" sheet can be cut with a sharp razor blade, but it is about as thick as you would want to go and cut by hand. This is the size I used for the swamps. It is very light and very thin which I loved. I sand and score the top with a razor blade to improve adhesion. This is the best size I found for the most uses. The .04" inch is very thin and can be cut with scissors or a lightweight hobby knife. I have used it for bases where I have a large building to provide rigidity. It is pretty floppy so its uses are limited, but it is the easiest to work with because it is so thin.
I paid around $10 dollars for a 2'x4' sheet of the .08" I found it a fair price for non-warping lightweight basing material.
http://www.usplastic.com/catalog/item.aspx?itemid=23330&catid=0&clickid=popcorn