Engine Idling problem?

I will try to start my car, to let it Idle and sometimes it will just keep shutting off it will not idle. This was intermittent until lately. Meaning I could go out and it would start and idle just fine. Lately it doesn't idle at all and I have to keep the accelerator pushed down slightly so it would not shut off. I can drive it, and it seems like it has good power without any problem, however when I come to a stop sign, the car will stop idling and I lose power to the engine including steering. What do you think it could be and how much would it approximately cost to correct this problem. Thanks, David.