Buying terrain is great. But I can justify an hour of my time to save some money.
Firstly, get your mind right. Keep an eye out for stuff that can be used. Packaging materials are fantastic, a great number of very durable materials that are designed to take a bit of stress and survive. I work near to the IT administrator of my firm and he's a great repository for interesting packing equipment. This firebase was originally a piece of Polystyrene protecting a large piece of server kit.
With a bit of work (in this firebase's case a lot) the shapes present in the material compliment the aesthetic that you overlay.
In this particular speed terrain example, cardboard corners were the mainstay. These come with almost every flat screen equipment these days, packaged and safe. (Image courtesy of Econanny)
Ingredients for the buildCheap paint! House paint can be bought in bulk, acts as a great base layer, and also can work to compliment your questionable building skills. I have black, and I have white. My terrain will be a cityscape so I rarely need anything more than Concrete colours.
Cheap glue! You know that really bad superglue that your friends bought you that you'd never dare use on your beautiful resin figures? Use that. Leftover quick drying cements? Use em up. Glue guns work great but might not be in every household.
Stuff! For the scenery itself, get your odd bits, your leftovers and your extraneous parts that you're hanging onto for that one special day. Its never going to come. Plates, wheels, cogs, levers, dials, sockets, pipes. Whatever you can find that looks utilitarian, use it.
For me, some golden resources come from the Guard Heavy weapon packs. Lascannon feeds and energy packs, mortar tubes, missile launcher mouths. Stands, handles, sandbags, rangefinders.
I made a basic bunker entrance with a scalpel, added a door, added some wiring and some battledamage. Done. Let it dry
Big brush. Thinnish coats(Strong sunshine and quick dry paint helps)
Dark grey all over.
Grey on topmost third of building.
Light grey on doorway lintels, and anywhere exposed to the elements. Done.
Pick it out quickly. Simple details will pop amongst the grey concrete. Flashes of red wiring. Bronze.Wash over to shade. Dry brush if needed.
Dab dab dab with a sponge for random effect. Black marks on areas most frequented.
Slosh. It'll dry later. A couple of snipped posters. Tamiya red and Abaddon black for Blood pools. Done.
And the best part about this thing is that it stacks! Flat pack function yaaay.
I've been bitten with the terrain bug as I want to try some Cities of Death games. And I've just got my hands on some balsa wood.
Bring on the shanty town. Hope this helps.