How much it costs to change the timing belt and where (toyota corolla 2000)?

I just fixed my car it had a blown head gaskit and now it's working good but I have a random misfiring in Cly 1234 and the mechanic told me that I need to change the timing belt because it hasn't been changed before and the car has 181k mils on it , could that be the reason for the misfiring ! Or he might setup the timing belt in the wrong position when he was fixing the head gaskit !! ( I have changed the sparkblugs and he checked the cols).and the car runs a little bit rough when I start it in the morning I have to press the gas petal till it worms up or the car will stall after the car worms up it works fine ( there is also high rmp that I can hear when the car on park or N)
One of the codes that the computer was shown was catalytic converter inefficiency
I am from Tampa Florida