POLONIUS: Costly thy habit as thy purse can buy, But not express'd in fancy.

SARAH: Polonius instructs Laertes to buy the best clothes he can afford, but not "fancy" items, that is, not trendy styles. In Elizabethan times, just as today, those who were footing the clothing bill suspected that designers were secretly bringing in new trends in fashion, just so that their clientele would purchase new clothes, even though the clothes they were wearing were still in perfectly good condition.