The formula for calculating the cost of an exacta box is (horses in box x (horses in box minus 1) x dollar amount of bet). A $2 box of two horses would be (2 x 1 x 2) = $4. A $2 box of three horses would be (3 x 2 x 2) = $12. A $2 box of four horses would be (4 x 3 x 2) = $24.