My 1997 Honda Civic Ex idles between 1500 and 2000 rpms?

My car is a manual and it is always between 1500-2000 rpms when I put the car in neutral, I could be driving it for an hour and it would still be idling between 1500 and 2000. What is the reason for this and how much would it cost to fix?

You could have a loose or bad sensor, Mass air flow comes to mind, as does the idle air control valve. Does it do it constant or is it a surge rev up and down action? Answer in comment section please.