A totally unique way you might note that the wind must carry the rain towards you and any amount of negliable moisture will vary the temperature and using a wind sock and timing the temperature as it drops due to rain moisture and calculating this out of the sun's light (Which would vary the teperature) could tell you according to the time of day and the increase or decrease in velocity of wind and how fast the temperature is changing (which is different and more rapid than wind or time of day) might give you an indication cheaply to predict when the rain is coming mearly by the ups and downs of the wind in coordination with the ups and downs of the temperature.
Also, you could use a simple litmus test strip type paper that changes coloe due to any moisture changes and gauge atmospheric conditions by the color change since it would change from it's original color as soon as any type of liquid vapour hits it telling you either fog or rain is on the way.
Just three cheaper ways to combine that might let you know when you will actually be wet! Ha!
to use these cheap techniques you would simply fallow simple placement of these tools and collabarate them to be useful for out door conditions.
By the way... the litmus paper could be numbered too show how much moisture is in the air, but once full saturated you would need more (It only works in one direction; dry to wet).
Each number corrisponds to a moisture level.
The litmus paper is usually used to show chemical content of your urine and changes color when you pee on them.
They are very sensitive to moisture only I have put them under changing, clear water and they change in regular clean water according to temperature.
I just noticed that the more we analyze this, the more expensive the tools, but if we use these tools then it becomes an inexpensive rain gauge.
I would add a alarm tone (Also cheap) to the construction of these tools and the louder it gets the closer the range or another indicator of when too put away the barb-b-que!