When comparing FPGA to potential future competitors (ASIC or really any technology) you can't compute the purchase cost of the item purchased in the past against the cost of the item just purchased and compare the W:MH or $ ratio's directly.
You need to factor in what it's paid back in the interval between when you purchased it and when the future technology is made available.
If you pay $700 for a BFL now and in 6 months you've made $350 with it, then a nifty ASIC version comes out for $500, you aren't comparing a $500 device against a $700 device, you are comparing a $350 device against a $500 device, and run your calculations from that point.
A device generating money RIGHT NOW is worth an infinite number of devices that might generate money in the future... until you have both devices generating money RIGHT NOW, you can't really compare them directly. When they are both available, compare them at their current costs, not at their costs in the past and future. Lest, if you follow that logic, you can deduce that sASICs purchased in 3 years are more valuable and thus sASICs purchased in 1 year, since those in 3 years will be cheaper than those next year. However, that sASIC available in one year, being less valuable than the one available in 3 years is a reason to wait - in three years, that one available is going to be less valuable than the one available in 5 years, so wait until then. Repeat this cycle in 5 years.
This is a good perspective on the situation.
Unfortunately even the BFL units are still not shipping and may not be for another month. So although you can PAY for the BFL unit today, you won't be able to MINE with it yet. Still, once you can, the comparison can be made in the manner you've outlined.