Determining the 'value' of miles by referencing full-fare C or F fares is misleading. Instead, try to find the cheapest consolidator fare in a premium can for a particular route, the kind of fare sold as part of a cruise holiday, and you'll be on the right track. The idea that a F redemption is worth $15,000 is just naive (and the proof is in the price airlines are willing to sell the miles in the first place.)