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]]
