The decision would depend on the cost of the repair, the age of the dishwasher and the cost of purchasing and installing a new one.
The most common problem with a dishwasher is "it doesn't drain". Similar to washing machine a lot of people would think that the drain pump needs to be replaced. In reality, only one or two times out of hundred "no drain" problems the drain pump needs to be replaced. In most cases of "no drain" problem the dishwasher needs to be pulled-out and disconnected prior to pulling it out so the repair could take an hour or even longer. The repair cost should be in the range of CAD 200 - 300 depending of time spent.
The next common problem with dishwashers is water leak. It is usually caused by door seal, water inlet valve or one of the hoses damaged - usually the drain hose. The repair should cost CAD 250 - 350, parts and labor included.
The two above case scenarios are most likely the ones where repair is justified but if the problem is related to control panel or circulation pump then repair would cost 400 and up 600 or even more. This is the point where you have to consider the age of the dishwasher and the price of a new one. Most dishwashers today in use would last 5 to 10 years. The price of a new dishwasher would depend on material from which is the tub made, the features built into the dishwasher and the brand. Cheaper dishwashers have tub made from plastic and their price starts at around 500 - 600, but those which are stainless steel inside (and out) are a couple of hundreds of dollars more expensive, just to start with, and price goes to above 1000.