Adding up the Tate

A friend and I went to see the rehang of the Tate Modern yesterday. It's interesting, perhaps flawed in places, - whoever decided that it would be good to hang the futurists in a big gaggle high up on one wall should be taken out and re-educated, - but compared with, say, MOMA in New York, it's imaginative, focussed and thought-provoking. But more than that, it's free. MOMA is twenty bucks. Now suppose you were an economist working for the state of New York or the city or whoever. How exactly would you decide whether it was better, on purely rational economic grounds, for MOMA to be free or not?

Obviously it will cost money to open it for nothing, but if you do, lots of people will go who could not or would not afford $20. And some of them will be inspired to create things, some of which will sell for cash. So making MOMA free will generate some extra economic activity. But how much, compared with the costs? How could you estimate it? While you are thinking about that, here's a picture of the silver birches outside the Tate, taken by my phone with its easy to fool exposure control.