We use PalmPilot (you can get old ones for $30 or so these days), and RallyCalc. It's not quite a $900 rally computer, but for 1/30th of the price, it eases the pain on the navigator significantly and can do most timing calculations. Otherwise, a combo of a cheap digital wristwatch and car's clock.

- Mind you, I abuse my car on the outside during the rallies... I find that if the driver and navigator can resolve their differences amicably, without throwing clocks etc at each other, things tend to last quite a long time on the inside of the car...  

Don't know why any modern clocks wouldn't take the abuse very well. The RallySport Ontario clocks go out in all kinds of weather and I'm sure they aren't always treated gently,  but never seem to fail.

If possible,  try to get an "atomic" clock and one that will display 24 hour time.  You should not have to pay more than $40 for one and if you watch for bargains,  you can often get them for much less than that.