Solution: (1) If the area planted with turf is x mu and the area planted with trees is (30-x) mu, then:

solve

A: The minimum area for planting turf is 18 mu.

(2) From the meaning of the question: y = 8000x+12000 (30-x) = 360000-4000x, and when x=20, y has a minimum value of 280000 yuan.

The relationship (1) is as follows: turf planting area ≥10; Tree planting area ≥10; Turf planting area ≥××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××

(2) Total cost = total cost of planting turf+total cost of planting trees, which is solved by the value of independent variable in (1).