I've stumbled on a wonderful website called Price of Travel which does exactly what its name implies. This site tracks travel prices for cities around the world and posts the results in an easy-to-read-and-compare format. Additional information about the city, its attractions, and museum costs are also just a click away.
In the Price of Travel's own words, here are the items they price for each city. Note that all costs are based on two travelers sharing a hotel room and taxis.
One night in the cheapest 3-star hotel available with a good location and mostly positive reviews. Hotel prices are per person based on double occupancy, so the full nightly rate is double what is listed for each city here.
Two 3-kilometer taxi rides per day (one ride paid per person)
One cultural attraction, such as a popular museum, per day
Three meals per day using the top end of the range of our “budget meals” for each city
Three beers (or wine) each day as an “entertainment fund.” Non-drinkers might have dessert and coffee or attend a local music performance instead, so this is a general benchmark that should be proportional for each city.
David and I will be traveling in Italy for $85 a day which is $30 less than the price listed on the site because we're staying in apartments, not hotels. But, still, this is an invaluable site for getting a general idea of daily costs.