The margin requirement for a short butterfly is the margin required on the credit spread portion of the strategy (difference between short and long strikes).

Example:

Sell to open 3 Mar 75 puts at $1.50

Buy to open 6 Mar 80 puts at $4.00

Sell to open 3 Mar 85 puts at $6.00  
 
Since the credit spread portion of the strategy is $5 wide, the margin requirement for this position is $1,500 ($500 x 3 contracts).