functional indexes and their costs

refer to the following 2 querys which were run after a vacuum analyze...

=# EXPLAIN SELECT * FROM sales_personal WHERE last_name='hansen';
NOTICE: QUERY PLAN:

Index Scan using sales_personal_last_name_index on sales_personal
(cost=0.00..280.68 rows=81 width=618)

EXPLAIN
=#EXPLAIN SELECT * FROM sales_personal WHERE lower(last_name)='hansen';
NOTICE: QUERY PLAN:

Index Scan using test_lower_idx on sales_personal (cost=0.00..5827.83
rows=1642 width=618)

EXPLAIN

anyone have any idea why the cost of using the functional index is so
much greater than the cost of using the regular (unfunctioned) index on
the same column?