Answer
$123,454$ dollars.
Work Step by Step
Step 1. Given $a=240,b=300,c=420\ ft$; we have $s=\frac{a+b+c}{2}=480\ ft$
Step 2. Using Heron's Formula, we have the area
$A=\sqrt {s(s-a)(s-b)(s-c)}=\sqrt {480(480-240)(480-300)(480-240)}\approx35,272.7$
Step 3. The total price is $3.5A\approx123,454$ dollars.
