what I've been quoted to upgrade from single to double glazing is a doubling in price (for a two story that's approaching 20k) in Sydney or above I'd probably just go 6.38mm glass instead
It's easier than that, simply look at your heating and cooling bill... for cooling you would need to incorporate low-E in the double glazing. If you don't have a heating or cooling cost, you don't need double glazing, and you only need double glazing in proportion to that cost.
That is leaving out the acoustic and comfort benefits.
But if your problem is solar heat gain, then shading is your number one method of dealing with that, followed by low-E.
Ed
Pretty much...
when I looked at the sizes of some of our windows it was also not possible(that I saw) to get double glazing in the long thin panes that we have without splitting it into multiple windows (we have some 2.4m wide x 60cm high windows)