To find the rate of change (slope) of the cost of broccoli in dollars per pound between the points (3, 8.25) and (6, 16.50), we can use the formula for the slope \( m \):
\[ m = \frac{y_2 - y_1}{x_2 - x_1} \]
Here, \((x_1, y_1) = (3, 8.25)\) and \((x_2, y_2) = (6, 16.50)\).
Plugging in the values:
\[ m = \frac{16.50 - 8.25}{6 - 3} = \frac{8.25}{3} = 2.75 \]
Thus, the rate of change in the purchase price of broccoli is:
\[ \boxed{2.75} \text{ dollars per pound} \]