Now everyone knows that girls cost a lot of money and time.

Girls = (money)(time)

And the old saying is that time is money, so the equality in reverse must also be true

time = money

Girls = (money)²

Everyone knows that money is the root of all evil

Money = (evil)^1/2

So now,

Girls = ((evil)^1/2)²

Which of course means that

Girls = evil

Pretty simple now that you think about it. But girls are still better than the alternative.