The volume of the slab is 100*100*.5=<<100*100*.5=5000>>5000 cubic feet
So it weighs 5000*150=<<5000*150=750000>>750000 pounds
That means the cost is 750000*$0.02=$<<750000*0.02=15000>>15,000 per house
So the total cost for all 3 houses is $15000*3=$<<15000*3=45000>>45,000
#### 45000