Valuation differs from area to area. Its like buying a unit in Chatswood vs a unit Mount Druitt. Same size, different price.
But i do agree that building a granny flat for an equity play is not necessarily the best move.
Interesting, but not all buildings (whether its granny flat or not) may not be feasible to build on slab and bearers & joists are the only alternative... seems a bit unfair.