Our costs were almost identical at $12,200 for the Bridgeport 35 (so almost the same floor plan). We also got the portico and al fresco included in the H-class slab, but our extra cost was due to the back of the house being extremely close to the sewer and needing piering. Like you, we thought it was reasonable...due to the amount of fill on our block it could have been much worse.
People have to realise that every house is different. Soil type, slope, fill, easements, setbacks, size and shape of the house, drainage requirements, single or double storey, and more will all impact on site costs. Some builders are definitely cheaper than others, but they may be more expensive in other areas or have less inclusions, so you have to look at the overall picture.
And the way we looked at it was that we preferred to pay for an upgraded slab and piering than have to deal with extensive cracking due to movement a few years down the track. Our old house (only one suburb away), built in the 80s on 600mm stumps, moved so much with variations in soil moisture from one season to the next, that we were forever filling and painting big gaps inside, and had a massive crack in the brickwork across the back of the house too.