Yes the cost factor is a big one. Both IOC and the Commonwealth Games Organization are looking at ways to reduce the overall costs. Its one of the reasons we are seeing the IOC look at former host cities and regions. I do not see a lot of money being used to build new facilities, though it would be nice.We also have more existing facilities from having hosted it before, so cost wise it would make more sense.
It probably is a better experience for the athletes and visitors to have the event focused on one place, easier to coordinate and more efficient cost wise too.