help understanding the bitmap heap scan costs

Hi all,
I don't fully understand how is the cost of a bitmap heap scan
computed. For instance when the explain output node is similar to the
following:

 Bitmap Heap Scan on test  (cost=17376.49..48595.93 rows=566707 width=6)
   Recheck Cond: ((text1 = 'A'::text) OR (text1 = 'C'::text))
   Filter: (num1 > 1)

how is the cost of the node (48595.93 - 17376.49) computed? I think it
should be something like:
(reltuples * ( index_filtering_factor_A + index_filtering_factor_B) )
* (cpu_tuple_cost + cpu_operator_cost)
+ (reltuples * ( index_filtering_factor_A + index_filtering_factor_B)
) / tuples_per_pages

but this does not equal the optimizer cost, so I guess I'm doing
something wrong. Suggestions?

Thanks,
Luca