The Specialty Coffee Association standard is 1:2 (18g in, 36g out) in 25-30 seconds for a balanced shot. Ristretto (1:1) is shorter and sweeter. Lungo (1:3) is longer and more acidic.
A single shot is one dose. A double shot is your dose pulled to a 1:2 yield. Assumes minimal waste. Actual cost varies with water, electricity, and equipment depreciation.