I don't speak for anyone else, but the base can be anything sturdy that fits the area I want to depict. My last is on a 8" x 16" corkboard-in-a -frame from Hobby Lobby= $2.99 on sale. They also sell unfinished wood for plaques that make good bases. Cheap. When my wife (the real builder in the house) is making shelves, I sneak around behind her and steal her scraps. Sometimes the piece of wood I find will determine the diorama, from single figure to a small section of a battlefield.
As far as road and terrain, there's always Celluclay. But for a fraction of the cost, here's what I do. I go to a park with about 6 big ziplocks. I double-handful loads of 6 different colored earth dark to light into them, along with twigs and foliage from different trees. Then, at home, I filter down the different soils. I start with an (old, if you wanna stay married) spaghetti collander (leaving 1/35 scale boulders), and filter down through a system of smaller sieves until the smallest yields 1/35 scale soil. I bought a 3-pak of different sized sieves at a Dollar Store, Baggies, too. Bag each "catch" from each size sieve. I end up with baggies labeled Rocks, Course, Fine, and Sand.
Using a combination of the latter three and some 50/50 glue/water (I usually blend colors and lean heavily on the Fine size), I mix to a thickness that willl somewhat hold a shape and spread with a spatula. I add small twigs to the mixture to simulate exposed roots. It won't take that much. Start with smaller batches and mix more if you need to. Save leftovers in an airtight plastic container (can be reconstituted if used fairly soon, but if they dry up ito a rock, no problem. Also sold in Dollar store).
When the spread is on the base and drying, I sprinkle the appropriate colored Sand where the paths and roads will be. Place Rocks if you want some small boulders lining the road, etc. I run old tires mounted in a brass rod to leave track depressions. A good tip I heard and use for multiple tank tracks is rolling a Legos wheel several times back and forth. Not Shep Paine quality, but will suffice for the first couple of projects. Also, the vinyl track from the tank you might be displaying- the mixture washes off easily.
After a day's dry, I shake off excess (save if you want, but it was free) and then scrub with an old toothbrush with varying pressure to blend the colors a bit. If I am left with too much sandy "sparkle, I might dab on some matte varnish with a brush. However, the grass planting in the next stage will usually cover the shine. When you get to the point of planting grasses and you want some thoughts, I'd be happy to share what I know with you. But, I'm no expert.
This is but one of many different methods, and it is FAR from being my invention. Yet it is no doubt the cheapest and easiest I've used. It will get you started and soon you will improve on the basic concept. It will be important to try new, challenging methods or eventually all of your bases will look the same. And that sucks when you've spent a lot of time working the models. Hope it helps.
SteveM