The most polite thing would be a phone call asking after them. Say that you got their message but that, unfortunately b/c of work commitments, you won't be in town when they are visiting. Give them your regrets, say it would have been nice & perhaps next time you can get together if the opportunity arises. Wish them a pleasant trip. Perhaps give them some suggestions for where to visit when they are there.

If cost is an issue, simply send a polite email saying the same thing.