Let me weigh in on one other thing. While, I think that the CPM (cost per mile) is very important in viewing how you value the mile you earn, you need to decide what type of earner you want to be.

Are you going to do everything possible to earn cheap miles?
Are you going to be someone who will divert purchases to earn miles?
Pay a little more for miles?

To decide this, you need to decide what your priorities are. Those factors include, among others, money, time (connections/Nonstops), program (CO,AA,UA...), objective (upgrades/free travel, international/domestic...), diversity among programs/consolidation in one, using miles for last minute travel or planning ahead, the list is long.

On the cost issue and using dialup service as an example: You can get 56K service for $5.95 per month: (www.access4less.net) w/o miles. So @ $16 per month, 25,000 miles would be equal to $400 of money I spent. To me, just not worth it I'd rather have the cash.

That said, I have determined that some things are worth the additional purchase and others not. I will do a mileage run to keep status or for reasons that are too good to pass up (See:LP million mile bonus program or triple miles on a cheap fare). I'll fly IAH-EFD in Houston to get extra segments on CO and there may be a small nominal cost to do that.

I think that two basic rules of thumb that will work are:
1) If the costs are equal then always take the miles. -and-
2) If you are already doing something and you can earn miles for it, get them. (See Idine and long distance.)
Thereafter, apply your own filter to the value of the miles.

I know that we FT's have discussed this on this board before. When searching for the discussions remember, some are diehards (4 day stay and stay in different hotels each night), some will get miles for everything they can without much additional effort and some just want to get all of the bonuses they can without any additional effort.

Just my 2c. I hope that some of this is helpful.
BSL