[barredo]

Apple's simple take on this may be revealing:

  iSomething   = $X
  iSomething+  = $X+100
  iSomething++ = $X+200

[lukifer]

I've always wondered about the sales ratios of iSomething+ versus the other two. It seems to be that most people either want the cheapest, or the best, and that the mid-range exists primarily as a stepping stone to convert the former buyers into the latter.

[alex_c]

I think many people want the second-cheapest (psychologically: good value, but not the worst), or the second most expensive (high quality, but not extravagant).

[ToastOpt]

This is especially true when "iSomething" is free. A lot of folks will want to 'buy in' to the software, out of guilt or gratitude, so will pay for "iSomething+" even if the delta is trivial.