For each type of shirt, he needed to buy, 40 employees / 4 = <<40/4=10>>10 shirts.
White men's T-shirts cost $20/shirt * 10 shirts = $<<20*10=200>>200.
White women's T-shirts cost $5 less, or ($20/shirt - $5/shirt) * 10 shirts = $15/shirt * 10 shirts = $150.
Black men's t-shirts cost, $18/shirt * 10 shirts = $<<18*10=180>>180.
Black women's T-shirts cost $ 5 less, so ($18/shirt - $5/shirt) * 10 shirts = $13/shirt * 10 shirts = $<<(18-5)*10=130>>130.
Now add up all these quantities to find the total: $200 + $150 + $180 + $130 = $<<200+150+180+130=660>>660.
#### 660