If your civ has access to a bunch of different luxeries, market places can be the best bang for your bunch in terms of making a city happy. Without a marketplace, it is 1
for every luxery that you have access to. With a marketplace the amount of smiley faces increases. for example the third and fourth luxery are worth 2
each. The fifth and sixth luxery are worth 3
each. As you can see those smiley faces can add up quick if you are lucky enough to have access to a bunch of luxeries.