My track was built on a very tight budget. Here's another super cheap tip.
The ground layer on my track is sand. It's a tip I picked up off a model railway page. When you travel, have some little containers in the car, or a compartmentalized tray in which you can collect interesting coloured sands or stones. Road cuttings are a great because they usually show a range of colours in the earth or rock.
I gathered some decomposing sandstone by the side of the road when we were out for a drive one day. When broken up they were sorted into rough colour piles and ground in a heavy mortar and pestle. Not difficult.

My motto: Cheap is good, but free is better.