"Cost" vs "expense" -- a usage question

While editing some ad copy, and I was given the sentence, "Defending a lawsuit can be a big cost for your business." (My italics.)

I keep thinking the proper word to use is "expense" rather than "cost" as in: "Defending a lawsuit can be a big expense for your business." But I'm just not sure I'm correct.

Can anyone help with more definitive guidance? Can "cost" and "expense" be used interchangeably in a sentence like this?

Thank you!

The noun cost could include more than money to be spent such as human resources (personnel who could work for other constructive things without the lawsuit), losing business to a competitor (business that can be generated without the lawsuit), etc. which are not easily and accurately monetized.

We call "the loss of other alternatives when one alternative is chosen" opportunity cost, not opportunity expense as it can't be monetized as easily as other expenses.

When you describe monthly living expenses, expenses is more idiomatic than cost as it could be monetized (almost exactly per month).

They could be used interchangeably depending on context, but the key difference is whether it could be monetized or not.

"Cost of living" is more idiomatic than "expense of living" because it is not easy to monetize it exactly.

In your context, cost is more idiomatic because it includes other things that cannot be monetized. It doesn't necessarily mean that you can't use cost for things that could be monetized.

First, a disclaimer: this is not legal advice. It's just a discussion about the English Language and its Usage.

The words cost and expense are synonyms, but phrases like can be a big expense are normally used in the narrow sense of the amount of money to be spent. The parallel construction can be a big cost, however, is commonly construed more broadly to include negative impacts to reputation, time, relationships, etc. (as well as money).

Expense noun 1 The cost incurred in or required for something - ODO

Cost noun 1.1 The effort, loss, or sacrifice necessary to achieve or obtain something - ODO

The difference between cost and expense is also usually related to magnitude. Expenses are usually money-related, while cost may not be. You'll see the word "cost" in phrases where "expense" wouldn't make sense:

They lost the game; that was the cost of the penalty.

The cost of living won't leave much of my paycheck.

There's no hard and fast rule about when you should switch from expense to cost in terms of monetary amounts (one person's expense is another's cost), but cost implies additional losses besides just money, and is appropriate when you're suggesting that more than just money is at stake, such as livelihood, possessions, or even life itself.