How much do I really need an Idle Control Valve for my car?

I have 169,000 miles on my Mitsu Galant and the engine light just came on for the first time. The next day the rpms started runnin from 1500 to 2500 when I had it in N or P. My mechanic said the "Idle Control Valve" is what I need and it may run anywhere from $250 to $300 ducats. So what's the worse that may happen if I just keep on driving without replacing this part? Those of you with a vehicle more than 5 years old or if you had to deal with the same thing - please respond.