Custom operator class costs

Hey!
I'm having some trouble optimizing a query that uses a custom operator class.
#Postgres has given me a solution for natural sort -
http://www.rhodiumtoad.org.uk/junk/naturalsort.sql

I'm trying to run it over a huge table - when running it on demand,
the data needs to be dumped to memory and sorted.

Sort (cost=31299.83..31668.83 rows=369 width=31)
Sort Key: name
-> Seq Scan on solutions_textbookpage (cost=0.00..25006.55
rows=369 width=31)
Filter: (active AND (textbook_id = 263))

That's obviously too slow. I've created an index using the custom
operator class, so I don't have to do the sort every time I try to
sort.

Index Scan Backward using natural_page_name_textbook on
solutions_textbookpage (cost=0.00..650.56 rows=371 width=31) (actual
time=0.061..0.962 rows=369 loops=1)
Index Cond: (textbook_id = 263)
Filter: active

Obviously a little faster!

The problem I'm having is that because operator classes have a low
cost estimation pg missestimates and tries to do the sort on demand
rather than using the index.

I can get pg to use the index by either jacking up cpu_operator_cost
or lowering random_page_cost. Is this the best way to do that, or is
there a smarter way to ensure that pg uses this index when I need it.