Well, if its near the bottom fo the bag, duct tape at +2 inches from the tear, over the tear, and another 2 inches below the tear will hold it. My favorite bag (old, cheap-ass Everlast canvas 75lber) tore in a very similar manner, and Duct tape has held it together for 5 years without reapplication.
That being said: bags are chap as shit. Check out
www.ringside.com ,
www.combatsports.com , and
www.boomersmma.com for heavybag deals.