Just to add to the growing discussion about Costco... St. John's retail is a very unique retail sector (as Kwajo mentioned) for a number of reasons, the biggest being we are an island and quite isolated from rest of the retail sector in Canada.
Retail in the St. John's is not limited to residents of the metro and surrounding area. Many people from Clarenville (approx 1.5 hours drive) east come to SJ on a regular basis to shop/dine/play etc. This actually pushes the catchment area up around 300,000+. As for Costco, this number is bigger since people from all over the island will come to shop, for both personal and business reasons. Someone from Corner Brook may only come in twice a year to shop at Costco but is very likely to drop a large sum of money. I have family from Central that come in with a large list of items to pick up at Costco for themselves and other family members and friends - further evidence to justify J_Murphy's claim...
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So is there large enough demand to justify another Costco on the island? On the surface is does seem so. But as Arrakis mentioned nothing is certain until we here it officially.