Reverse Commission Calculation

       ----A---- ---B--- ------------------------------------------C------------------------------------------
   1      GP      Rate                                                                                        
   2                     Blank row required                                                                   
   3   $0            25%                                                                                      
   4   $1,000        50%                                                                                      
   5   $2,000       100%                                                                                      
   6                                                                                                          
   7      GP      Comm                                                                                        
   8   $0             0  B8 and down: =SUMPRODUCT((A8>$A$3:$A$5) * (A8-$A$3:$A$5) * ($B$3:$B$5 - ($B$2:$B$4)))
   9   $1,000       250                                                                                       
  10   $2,000       750                                                                                       
  11   $100,000  98,750                                                                                       
  12                                                                                                          
  13     Comm      GP                                                                                         
  14   $100      $400    B14 and down: =PERCENTILE($A$8:$A$11, PERCENTRANK($B$8:$B$11, A14, 6))               
  15   $200      $800                                                                                         
  16   $300      $1,100                                                                                       
  17   $400      $1,300                                                                                       
  18   $500      $1,500                                                                                       
  19   $600      $1,700                                                                                       
  20   $700      $1,900                                                                                       
  21   $800      $2,050                                                                                       
  22   $900      $2,150                                                                                       
  23   $1,000    $2,250                                                                                       
  24   $1,100    $2,350                                                                                       
  25   $1,200    $2,450

If the Commission target is placed in cell F9, then:

=IF(F9>=750,(F9-750+2000),IF(F9>=250,(F9-250)/0.5+1000,F9/0.25))

will provide the GP required to earn it.