How much does it cost to bet a five dollar trifecta box?

Where X is the number runners you want to box and $ is the amount you want to spend on a single bet (in this case $5):

X*(X-1)*(X-2)*$= cost of box bet

For a 4-runner box in a $1 wager this would be 4*3*2*$1=$24

For a $5 bet that would be 4*3*2*$5=$120