In a manner that involves considerable cost or expense.
Formed by adding the adverbial suffix '-ly' (from Old English '-lice') to 'expenseful.' This creates an adverb from an adjective, following standard English word-formation patterns dating back to Old English.
By adding '-ly' to 'expenseful,' English speakers can turn the adjective into an adverb—but it's so uncommon that most people would just say 'at great expense' instead. This shows how English has more potential words than we actually use.
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