In our new house hubby poured thin layer of concrete over existing slab in ensuite and main bathroom, embedding coloured stones in it. It's been polished and sealed with crushed glass added just to the shower area .... we definitely won't slip on it, but will have to use a soft broom to clean shower area, as it would shred the pad on my mop (we haven't moved in yet).

You've posted on Australian Houzz. Not sure you meant to do that.

Adjustable shower bases solve the ongoing problem of drain pipe misalignment by having an adjustment zone.

Well the tiles need to be laid on something but an extra $3k without discussion is crazy. Was the area already waterproofed and ready to go? Is it a renovation that you’re managing yourself? What is the total cost of the tiling job? What works were specified in your quote? I would expect tiling costs to be about $120 per metre depending on the tiles and area so this may be a case of paying peanuts and getting monkeys. Apart from the price hike are you happy with the job? You have limited options: pay the tiler, negotiate a better price or refuse to pay and see what they say/do. To assist with option 2 and 3 you may need to pay a building inspector to inspect and report so you have a professional opinion.