I'm glad you posted more information, as it shows that some of my assumptions were definitely invalid.

It sounds like your company has come to its senses as to what a full-fare round-trip ticket should be considered to cost. If I read your response correctly, they used to consider it the one-way fare multiplied by two, but now consider it to be the actual round-trip fare. If that's the case, I think they've taken a step in the right direction.

Taking your example, you state there are two fares, based on minimum and maximum stays. Well, I would think that what you would take to be the "full-fare" would be the fare based upon the amount of time you are planning to stay. If you have a planned travel date and a plan return date, I would consider the maximum full-fare to be the highest Y fare you could find for those dates.

I could be wrong, but it sounds like you're trying to get a "vacation allowance" equal to the maximum possible fare you can find for the dates you are traveling. You state there are two fares for the dates you wish to travel: $4530 and $8898. Are you trying to book the $4530 fare and attempting to justify recompensation for the $8898 fare? More power to you if you can make some money off your vacation, but I think ethically what you should claim for you vacation allowance is the amount that you actually spend for your travel, whether it be the $4530 fare or the $8898 fare.