The cost of the planks for one birdhouse is 7 * 3 = $<<7*3=21>>21.
And the nails are a cost of 20 * 0.05 = $<<20*0.05=1>>1 for each birdhouse.
So to build one birdhouse one will need 21 + 1 = $<<21+1=22>>22.
So the cost of building 4 birdhouses is at 4 * 22 = $<<4*22=88>>88.
#### 88