The best guide you will get to future expenditure is to compare buildings of a similar size and facilities that are maybe four or five years older. That said there are so many variables, from the quality of the original work to where the installation sits in the building and the effect of prevailing weather that you are never going to get 100 per cent accuracy.
Regarding whether or not you include GST in the quotes, it sounds like the surveyors are trying to minimise the cost assessment in every way possible. Since GST is unlikely to change any time soon, I’d include it.