Unsurprisingly, rocks are a vital component of a Zen garden. The least expensive option is to just find some rocks lying around in the world.
The least expensive option for the sand element is to use play sand – you can get 50 lbs. for less than $30.
You could make your own water fountain that’d be much larger. The instructions linked here use plant pots. Provided you can find a cheap set, this should cost you around $50.
A bamboo privacy screen serves 2 needs by blocking off your space and adding bamboo. It’s not necessarily super cheap, but it doesn’t need to be expensive.
These really don’t need to be expensive, and you’ll find that resin statues are the cheapest. Helpfully, resin will last for ages outdoors and is very lightweight.