Peformance question TKPROF/Autotrace Cost High when using index why?

I'm going through a legacy database application to do some application code performance tuning of the PL/SQL.
I am constrained from changing the code logic so I can only add indexes and hints. As I identify full-table-scans I'm using hints to force the code to use the index and now the Cost is Higher than with full table scans. Though the application code runs in batch mode and processes a lot of records.
Can anyone provide any insight into why the cost goes up when using Hints in the SQL?
I have additional testing to do though I just wanted to pose the question.

Firstly, the cost can be less for a full table scan because with a full table scan Oracle can do multi block IO with a full scan. So reading a large amount of data from a table will go much faster if the table is full scanned as opposed to accessed by an index (your forcing the query with a hint can actually be extremely detrimental to the query).
Secondly, if you are not allowed to make any code changes, i'd recommend not taking the path you are currently on (adding hints). That may be warranted in a few situations, but likely not many assuming you have accurate statistics set up on your database.
Here's an article that deals with full table scans ....
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:9422487749968
Hope that Helps.

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Peformance question TKPROF/Autotrace Cost High when using index why?

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