My car started to run sluggish when.

My car started to run sluggish when excellerating, and rough at the idling, I went to auto zone, put the code machine on, it came up with a cylinder 2 problem code, I then had a friend put in spark plugs, I had the coil cap checked out for that clyinder, can't seem to fine the problem, I was told it could be a vacume leak, I even put gum out in it with the right directions on how to do that, could this possible be a vacume leak? What would the cost to repair it be?
Thank you
marshfield8@verizon. Net