That's another valid reason, but it doesn't allow to dismiss factor of cost of employee entirely.
Company, I work for, has office in capital city, and offices in smaller cities. Talent pool of capital city would easily accommodate us. Other offices exist because it's cheaper to hire there. If cost would be the same, they would simply cease to exist. Cost difference in means of the building is less than 5% of overall monthly budget, so, while helping it's not deciding factor.
