Questions for anyone that knows about cars?

I know nothing at all about cars. This info was told to me by a friend of my husband's that's pretty car savy. So my car (2005 Scion Xb) is idling too low (about 500 or less RPMs when at a stop) and it's making my car jump/pause/whatever you wanna call it. It's worse when I have the AC on. While I'm driving/in motion it's totally fine though. I'm taking it to the dealership at noon today. Any idea how much this would cost to fix? Is this something major or minor? Any and all input is appreciated.