Slab upgrade costs - M class to E class

Can anyone please give me an indication if $20,000 is reasonable to be charged for an upgrade from an m class to an e class slab for a house with a floor area of 299m2?

We are also being charged an extra $20000 for pierings, so am some what in shock to receive a $40000 variation!

we've been quoted anywhere between $13k to 18k extra for E class soil for a 280m2 house so I think 20k is a little bit on the expensive side but not outlandish. Pierings depend a lot on whats underneath, are they piering with concrete or screw piers?

We were quoted 15k for screw piering because our soil doesn't go down to rock so they use screw piers, but bore piers are probably a bit more expensive but they put them down until they find solid rock normally. so again I'd say you're within a reasonable range. depends on how bad your soil is and what's underneath.

Reasonable range? Based on what? Geotech report, bearing capacity, materials Quantities, Builder profit?
These are questions you should be asking before you hand over a deposit to the builder
as one thing is for certain , the price you receive before, will always be less than the variation after you've given the builder a deposit to start, you needed proof that the initial PS wasn't a stab in the dark or worse an enticement to sign and hand over a deposit
The later is a case/complaint for Fair Trading.hth