As I do a lot of building in schools as part of social studies curriculum, I've had to be creative as how to keep costs from going through the roof. Teachers dont make enough money.
One of the best things I've learned is that one man's waste is another man's treasure. Specifically when that waste is terrain building materials.
Latex house paint is great for painting buildings, hills and groundwork, but at 30 dollars a gollon, not really that cheap. Not when you want several colours anyway. Possibly not a problem for individual builders, but when 30 kids are making a board, you need the paint.
Solution: Mistinted paint in your local paint store. People buy the paint, find it doesnt look quite right, then return it. paint store sells it off at 1/4 to 1/3 the original price. Big boards just got a lot more affordable.
Another great source of material is the local recycling center. You may have to ask permission to go and grab stuff, in fact that proably a very good idea if its any bigger than a local curbside depot.
once you get the ok, go to town. Styrofoam is going straight to the landfill from there, unless you liberate it, carve it up, and paint it with your already obtained cheap latex paint.
Another nice feature is that packaging styrofoam is often cast into circular and dome shapes, which gives you some nice chances to be creative with your buildings.

 
There probably is no perfect glue for every application. I have used many different types over the years, from white school glue to super epoxy resins.
For most uses I prefer Weldbond brand white glue, which I buy at my local Rona hardware store. I'm sure any hardware store has it. It comes in small sizes all the way up to big gallon jugs, which I buy.
it has several good qualities that make it a preferable glue for hobby uses, in my opinion.
1) dries clear  - a big factor when you add glue after painting for whatever reason. some glues I've used dry noticeably yellow, and dont look that good.
2)dries fast  - but not too fast... I find I have more than enough time to move things around to get a perfect fit before I need to worry about clamping or otherwise locking the pieces into place.
3)water soluable  - you can mix this glue with other water soluble products like latex paint or plaster, or both at the same time, to make a super strong pigmented, spreadable substance.
 
As a terrain builder, I enjoy a varity of different aspects of the hobby. From an artistic point of view, its fun to create something out of an idea and have it used by people all over the world. As a gamer, I get to put terrain on peoples tables that I want to use myself. Its a win-win scenario.
Its always tough to decide what type of work to do though, as there isnt always a complete transfer for game systems. Warhammer 40K, being a 28mm scale game, created buildings that dwarf Flames of war 15mm minis.
On the positive side, tree stands and hills work pretty well for both.
Stylistically there is a difference as well. I try to be historically based for my Flames of war buildings, but put more of a "future" feel to the 40k items.
There's a lot of crossover in there, but the scale is the big divider.
For those who dont know...
28mm refers to the fact that 6 feet in game size is represented by 28mm on the game table. The same is true of the 15mm versions.