At Repco NSW (and I'm told, other states as well) these tyre pressure monitors r on sale 4 $20 (was $69).

They r battery powered and once installed they show a red light when pressure is low, (I think it's 2 or 3 psi below the initial setting). Pressure can only b set once. This is done by very carefully setting the pressure for each tyre. When the monitor is screwed on 4 the 1st time the pressure is set at that amount. If pressure decreases below the set amount a red light shines.
Since the light only shines when pressure is low, the battery should last a long time.
What I don't know is what happens when the battery is low - does it indicate somehow. Don't know yet as I'm picking mine up 2moro and will check the web after that. In practical terms u should notice a flattening tyre and if the monitor is not red u know it's time 4 a replacement.
The good thing is u get 4 so they should last quite a while.