We are at the top of a hill and had giant retaining walls built for ours. We did a bunch of stuff at the same time so I don't recall what the garage cost on its own.
My advice would be to get the walls dug now before winter. Ours was done in the middle of that giant cold snap 2-3 years ago where it was -30 for most of January. They had to run ground heaters for a good week to get the ground unfrozen so they could dig and pour. That was a big annoyance as the machine ran 24/7 and had a very deep humming noise you could hear and sort of feel.
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