2005 Mazda 3 What's wrong with my car?

My 2005 mazda 3 had a very rough idle yesterday in traffic, along with a blinking check engine light, and nasty smoke coming out of the tail pipe. Today, the dealer mechanic told me the computer is blown & I must replace it. My car has 74,000 miles, isn't that a bit early for the computer to be dying?

The computer code is p0302 misfire on cylinder #2.

Should I spend $800 on the used computer & labor? They said even if I don't have the work done, I owe $160 for the time they've already spent on the car. I don't know what to do.