A southern bridge which will interfere with shipping and therefore is not considered to be ideal, will likely be well over $1 billion (by the time highways in South End Halifax are constructed it could be $1.5 billion - for example). Part of the highway for the southern bridge network envisioned in the 60's and 70's does not exist - Harbour Drive. Likewise a tunnel which would not interfere with shipping would be even more money and still require a highway network (or at least a major roadway).
On the other hand a twin MacKay Bridge might only be $500 million with 4 lanes of vehicular traffic and 2 lanes of LRT traffic. (This is just a guess based on the numbers being given for the southern bridge).
The difference in cost could be close to $1 billion dollars by the time the southern bridge and road network is complete. If a southern bridge can be easily accessed by 100,000 people near Shearwater, and a twin MacKay bridge can be easily accessed by 100,000 people near Shannon Park (say a radius of a few miles), then the difference in cost is $1 billion dollars/100,000 people = $10,000 per person. If there are 2.5 people in each household, then that is $25,000 per household extra. The southern bridge will also have higher ongoing maintenance costs. With these costs in mind, and the negative effect that a southern crossing might have on shipping wouldn't it be better to focus grow towards a twin MacKay Bridge instead of a southern crossing. Growth trends can be planned in advance along traffic corridors. Growth doesn't have to just occur wherever it occurs.
A lot of people can be fit into the county of Halifax without using southern Dartmouth. For example, the city of Toronto (not including all the suburbs such as Mississauga) has 2,503,000 in 630 square kilometers. The Toronto Urban Area has 4,753,000 people in 1749 square kilometers (source:
http://en.wikipedia.org/wiki/Toronto ). For comparison, the land area of Halifax County (HRM) is 5,500 square kilometers. If you just consider the densely populated Urban Area, the population is 283,000 in just 263 square kilometers (
http://en.wikipedia.org/wiki/Halifax_Regional_Municipality ). So you can see that Halifax could get up to almost 1,000,000 people in less than 1000 square kilometers (quite easily, the city of Toronto has 2.5 million in 630 square kilometers, about 1/10th the county of Halifax land area). So it is not necessary to have southern Dartmouth populated. Just concentrate the growth in areas that are the cheapest to service. This is already being done to some extend - HRM wants to add more people onto the peninsula.