Weekly subscription! You can buy clothes in the prison store for relatively cheap, and have them forever and wear them inside your own room or a lover's room whenever you want. It's wearing things in public that cost a four-day subscription fee.
Her current dress is expensive because it's very full-coverage and opaque, despite the holes. There's several budgetary steps down from there she could take before she went full Scantily Clad.
The bureaucrat notes that Opalyn doesn't presently have a book to take with her; and that Duchesses who aren't trying to maintain plausible deniability about being huge sluts would want to buy rights to some piece of clothing that didn't decrease their vulnerability--like thigh-high black stockings--to signal that they were leaving themselves vulnerable on purpose, not because they were low-status people who couldn't afford better.