What could be making the car knock and slowly accelerate?

I have a 93 Chevy Corsica I think V6 engine. It only has 120,000 miles on it and I just replaced the radiator. I was driving it at about 80mph the other day and then when I slowed down to stop at a light it started knocking and shaking really bad. It had a slight knocking before this but I didn't think anything of it because its an old car. It also was taking a really long time to accelerate. I explained what was happening to my dad and he said it was prob the spark plugs. Is this what it sounds like and if so how much would it typically cost to get them replaced at an auto body shop like Firestone or Good Year? I just don't want to get ripped off if I go up there.