Idle problem

I have a 1985 Toyota Tercel.

The problem is after I have driven it for a little bit, it stalls at stops. When I first start it up it idles a little high but stays on. It does however have 350,000 miles on it, but they are all highway miles and in general the car runs good.

Do you have any sugestions on how I can fix this and how much it will cost approximatly.

Thanks.