The BP Effect of a long iron condor is the net cost of the spread (cost of the long options - the credit of the short options).  

Example:
  
Sell to open 6 Mar 40 puts at $2.50
  
Buy to open 6 Mar 50 puts at $6.00
  
Buy to open 6 Mar 60 calls at $6.00  

Sell to open 6 Mar 70 calls at $1.50 
  
The BP Effect of this position is $4,800 [($6.00 x 6 contracts) + ($6.00 x 6 contracts) - ($2.50 x 6 contracts) - ($1.50 x 6 contracts)].