Generally speaking, the ID90 fare is 10% of full, unrestricted fare. Not full Y but the highest fare a revenue paying passenger would pay for a walk up ticket. In my experience, this usually shakes down to somewhere near 1/2 of the cheapest advance purchase you can get in most markets. Sometimes that works for you and sometimes it doesn't. As an example: I would never consider travelling nonrev between PHX and LAX because the fare is too cheap if you plan ahead BUT I would most definitely travel nonrev from PHX to LHR because I'm still paying considerably less than if I purchased a ticket with a lot more flexibility in my schedule. It's all personal preference. A little research goes a long way.