You can't just wave your hands and say "I think this number is small, so I'm going to call it zero". There's an infinite difference between any nonzero number and zero. And in fact, the economic value of an average piece of art is obviously not zero. I'm not going to bother to try to estimate it -- do any 3-year-old's scribblings count? -- but it's not zero.

You're also overlooking the rather obvious and important fact that the quality of different works varies tremendously. While it's true that appreciating art does require some time, the amount of value I receive in return, and therefore the price I'm willing to pay, is a function of (my perception of) the quality of the work. And it doesn't necessarily take me more time to experience a higher-quality work than a lower-quality one.