Seriously?
Sorry, I don't mean to be rude but I don't think I could be more surprised by a question.The answer is that styrofoam of one sort or another is pretty much the standard material used to make most kinds of terrain. From hills to bolder strewn fields to castle walls and wizard's towers. Its cheap, fairly easy to work with relatively durable, lightweight and easy to transport.
Many many people start with the styrofoam inserts that pad computers and telvisions in their boxes but you can get sheets of the stuff at Do It Yourself stores. They white kind is cheaper but also less durable. Generally speaking the best bet for terrain is the high density insulation styrofoam that is usually pink or blue.
Some advice for working with the stuff.
A hot wire cutter is a godsend for shaping it. You can cut it with a knife but a wire cutter is much MUCH cleaner and faster.
You can use basic white or wood glue to glue multiple layers together, like for making a hill, but sometimes the sheets will warp so it is best to weight them down until the glue thoroughly dries.
Spray paint or any kind of aerosol with acetone in it will dissolve the surface. Sometimes this provides a useful effect but most of the time it is bad. Spray paint that has keytones instead of acetones is safe though.
If you don't have a hot wire cutter, probably best to cut the stuff outside 'cause it WILL make a mess.
So, to actually answer your question,
Very.
Hope that helps.
