Another way is to use three Solve functions for find the intersection points of the set of lines. Then use cross product to pin down the area.

sol1=Solve[-x + 1030 == 400 - 0.1 x, x] (* x = 700*)

... (for other two lines)

Then extract the points with Replace + Rule combo.

point1 = {x,-x+1030 }/.sol1

...

Then you should be able to compute the vectors of two sides

point2-point1

point3-point1

Finally, use cross product to find the result:

(1/2*Cross[point2-point1, point3-point1])[[3]]