My automatic car keeps stalling. What is the problem?

I have a 2002 Dodge Neon and it keeps stalling when I stop or slow down. It doesn't do it every time I stop, but it does it pretty often.

Also, I called a car mechanic to see how much it would cost me to repair it, but he said that they would need to do a diagnostic test which would cost me $100 to start out with. That doesn't include the actual problem with the car. Is there any cheaper option for me that could fix the problem also?