This low number per person, compared to what on land visitors spend, is part of what is pushing port cities to limit or ban cruise ships. Having several thousand people descend all at once to crowd streets and use local services is not very appealing when they spend only a fraction of what other tourists do.
My cruise port spending, of course, varies but sometimes it's nothing. I may just walk around and see the sites. We are not big shoppers and rarely buy souvenirs. We do get a meal when we are on shore for a longer time. But we don't usually contribute much to the local economy.