Quick answer
Price per piece is the set price divided by its piece count, expressed in cents. A $79.99 set with 1,034 pieces is 7.7 cents per piece.
The number most collectors carry in their head as "fair" is about 10 cents a piece, and it is a rule of thumb rather than a published figure. Anything meaningfully below that is usually a good buy; meaningfully above it usually means you are paying for a licence, a small production run, or minifigures.
Two adjustments make the figure far more useful, and most calculators skip both. Work from the price you will actually pay rather than the shelf price, and take the minifigures out before judging the bricks.