If he's got on the phone to Holden and priced up genuine parts I can see how the mechanic came up with a quote like that. Anything for the LS1 seems cheap aftermarket but outrageous from Holden. Here's some of the Holden prices I was quoted when I did my lifters in 2012. I don't imagine they got that much cheaper over the last few years.
-The torque-to-yield M11 cylinder head bolts
Long x 16 11588291 $22.20ech
Medium x 4 12560745 $7.27ech
Short x 10 12558840 $14.05ech
-Head gaskets
12558809 $191.96
12558810 $231.99
-Intake manifold gaskets
12533587 $96.80set
-Exhaust manifold gaskets
12617944 $63.95ech
-Full set of hydraulic lifters.
17122490 $112.86ech
Instead I ordered the parts from summit in the US.
Genuine GM head bolts, gaskets, lifters, valve springs, exhaust gaskets, spark plugs and leads were under $600 delivered.