Terrain, it is ubiquitous with wargaming. It is needed, however, how far do you need to go? To me, there's three levels.
1. Basic- Basic terrain is whatever you happen to find laying around your house that can be slapped down on the table.
2. Mass Market- These are kits, either from GW, third party manufacturer, or from a model train company. They are basically big models.
3. Customized- Customized terrain is almost all scratch built, and depending on your ability it can look amazing or like crap.
Now, each of these levels has its own benefits and drawbacks.
Basic Terrain is dirt cheap, but it looks like dirt. Basic terrain is your basic horders fans. Shoeboxes, cereal boxes, Pringles Cans, old GW boxes, kids block sets, the Styrofoam that comes with electronics, and my personal favorite, the plastic half boxes that comes with ground meat at certain stores. Most of this stuff looks like crap. It can be slightly enhanced with a simple coat of grey spray paint.
Mass Market Terrain is expensive. That's its main drawback, and really the only major one. Mass Market terrain generally is sturdy, looks good, and paints up a treat. The problem with it is that it is often not designed for Mordheim, and difficult to get models into and out of. Looks good, but it might as well be a solid brick.
Customized Terrain is the best of all worlds. It can be as cheap or expensive as you want it to be. It can be as detailed as you are able to handle. You can use bits of mass market kits, or do everything from scratch. This is my preferred method.
My last bit of terrain I did, I did totally in a span of two days, with nothing but $10 worth of stuff from the dollar store. Next time, I'm going to go over the things that I bought and used, and some of my other favorite found things to use for terrain.