J is not quite a luxury product, so I don't think it correlates so well with income...there are many other factors as people have mention. I think you'll find better correlation with F.

As for me choosing between Y and J, I tend to look at the 'value' factor. Y if flight is quite empty and you'll most likely get a row to yourself anyways. J becomes 'value' for money usually at peak times when e.g. during xmas J only costs twice as much as Y (for a fully loaded flight), it would otherwise cost 3 times as much other times of the year.