The range of value you could assign to the miles wide because of all the variables involved. It really comes down to how much money do you save by not buying the ticket, would you have actually have paid that amount if you did not have miles, did you pay extra to get the miles (mileage run), etc. It is different for everybody. A couple examples from the past two years: I have 40,000 CO miles that I would like to use for upgrades, I often can't buy the lowest fare and get the upgrade so there is a cost that obviously has to be factored in to the calculation. Another factor is that CO often offers discounted first class fares now so I could just pay for the ticket at say $1300 rather than give 20,000 miles AND pay about $600 for the coach ticket. The 20,000 is really only saving me $700 ASSUMING I was actually prepared to shell out $1300 if that was the only way. On the other hand there is British Airways, on three ocassions I have cashed in 100,000 miles for first class tickets that would have cost over $10,000 each, and every time I got exactly the dates and flights I requested with less than 3 months advance planning. 100,000 miles on some airlines is much more valuable than others. There are many, many variables. You just have to consider your own situation.

Also, welcome to the board, if you fly often you have come to the best place to get accurate and thorough information.