If you want to buy this book $100 off then ...
--- Do I need "for"/"at" before "$100"?
Newguest If you want to buy this book $100 off then ... That certainly doesn't work. , "...
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NewguestIf you want to buy this book $100 off then ...
That certainly doesn't work. I suspect you mean, e.g., "... for $100 less than the price listed/advertised ..." Can you give us more of the context and a full sentence please?