I keep it simple: everything after-tax. If I'm ever going to voluntarily buy an FF mile (e.g., acquire one other than by taking a flight where there is no cheaper-but-otherwise-equal non-mileage-earning option), I'm buying it with after-tax dollars. Obviously I'm buying my personal paid flights with after-tax dollars. So I don't know why I'd allow pre-tax dollars to complicate the equation.

The reality is that, for the most part, I don't voluntarily purchase FF miles anymore. I've bumped all of my airline credit cards. I don't use airline partner vendors for shopping unless absolutely necessary. I don't buy airline miles from rental car agencies because I take Hertz Points instead. Right now, I'm mainly buying hotel points (from Amex and Chase), so I am very concerned about the value I am getting from those.

So since my accrual of FF miles is almost exclusively from flights on which there is no equivalent-but-cheaper way to fly without earning miles, I am somewhat less concerned about their specific value. It varies so much trip-to-trip anyway, and sometimes I burn 'em for international upgrades, for which I have no basis against which to compare because I've never actually bought a J seat with my own money to begin with.

So....I look at my most recent redemptions for trips I both (a) knew I would take regardless of whether I had miles, and (b) redeemed miles for a coach award, which is the class of service I would always buy with my own money.

- 15k UA to Denver on dates for which the lowest fare was $288 all-in.
- 35k DL to Belize on dates for which the lowest fare on any airline was $818 all in, but I had to pay about $55 per seat in taxes on this one.
- 25k NW to DTW, lowest nonstop fare was $475 all in, and it was a short trip for which hassling with WN on a Chicago connection wasn't a good option.
- 25k AA to YVR, lowest fares on any airline were in the high-$400's.

All of these were in the past six months. So I'm pretty comfortable with the fact that I get around 2 cents a mile when I redeem them.

The catch is that the miles sit in my account for a while before I redeem them. So in terms of buying them, I'd never spend even 1.5 cents these days. Those Delta miles were earned in 2000. Those NW miles are from the late '90's. AA and UA miles are fresher - I earn & burn more frequently there - but still they were 2-3 years old.

So it's complicated. Maybe I'd buy for a penny knowing I could redeem for 1.5 a couple years down the road. But these days, I like hotel points a lot better because I can more easily tie them straight to known values and redeem them within months of earning them.