Re:

how can there be 10k of sitework on five plams?

the costs that we post to site work are

building/deck
excavation
grade for slab/footings
base material in slab
compaction of base
generator bunker
excavation into hillside
grade for slab an retaining walls
base and backfill material
waterproofing /drainage

the $10,000 estimate covers all the above and at the end of the day I think there will be some finish work even though you said just gravel

break down this cost.