The first computations yield 
    1     10,000
    2     10,500
    3     11,000
    4     11,500
    5     12,000

    6     12,500
    7     13,000
    8     13,500
    9     14,000
   10     14,500

   11     15,000
   12     15,500
   13     16,000
   14     16,500
   15     17,000

   16     17,500
   17     18,000
   18     18,500
   19     19,000
   20     19,500

   21     20,000
   22     20,500
   23     21,000
   24     21,500
   25     22,000

   26     22,500
   27     23,000
   28     23,500
   29     24,000
   30     24,500
So 
p(10) = 14,500
p(17) = 18,000
p(27) = 23,000
The second computations yield 
p(10) = 10,000 + 500 *  9 = 14,500
p(17) = 10,000 + 500 * 16 = 18,000
p(27) = 10,000 + 500 * 26 = 23,000
Notice that the two computations yield the same numbers.
Copyright c 1995 by PWS Publishing Company, a division of International Thomson Publishing Inc. Comments to Frank Wattenberg, Department of Mathematics, Carroll College, Helena, MT 59625.