The value to you of one mile very much depends on how you use them. To find that value, you need three numbers:
Total dollar cost of the ticket you would buy with your miles: Be sure the dollar price of the ticket is for one that is similar in restrictions and advanced payment to the conditions on using the miles. Usually this is a discount ticket sold by either the airline or a consolidator. Be sure you have the total cost, including taxes and fees.
Total miles you would lose by using miles to acquire the ticket: This includes both the miles you pay to get the ticket and the miles you would have gotten by buying the same ticket with dollars.
The number of dollars you would have to spend anyway by acquiring the ticket with miles: You will pay departure, and excise taxes, security fees, and anything else your airline thinks it can get away with when you use miles to acquire a ticket. These charges can amount to over $100.
To compute the value of one mile;
Examples: A round trip to Europe from the West Coast of the United states can usually be purchased through consolidators for about $300 in the winter and $800 in the summer, plus taxes and fees. Yet both would cost 50000 miles on most airlines. Suppose the taxes and fees are $50 and the length of the round trip is 10000 miles. Further suppose that you have exactly 50000 miles in your account. If you use your miles to get the summer ticket, you would have 0 miles left in and be out of pocket $50. If you were to buy your ticket with dollars you would be out of pocket $850, but you would have 60000 miles in your account after taking the flights. So your net savings by using miles is $850 - $50 = $800, and your net loss of miles is 10000 + 50000 = 60000 miles. Divide those numbers. You are getting $800/60000 = 1.33 cents per mile. Yet if you use your miles to get the winter ticket, you are getting $300/60000 = 0.5 cents per mile. I would use my cash to buy the winter ticket, and save my miles for something more valuable.
A 5000 mile coast to coast round trip domestic ticket within the United States costs around $200 plus taxes and fees on Southwest Airlines if you wait for the right sale and are willing to fly on three different airplanes to get there and eat peanuts while doing so. At that price, assuming the taxes and fees are the same no matter how you acquire the ticket, using 25,000 miles for the same trip would value the miles at $200/30000 = 0.67 cents per mile. Better use those miles for something more expensive. But perhaps you want a non-stop flight and a meal, and figure that will cost you $320 plus taxes. Well, $320/30000 = 1.066 cents per mile. That's a little better.
It seems that the dollar cost of business class is always way over twice the cost of a cattle class ticket, yet the number of miles to buy the business class ticket is usually only 1.5 or 1.6 times the miles for cattle class. So if you usually fly business class and have lots of miles to spend, for you business class might be a very cost effective method of using those miles.
O.K., now go decide how you might use your miles. Then get a good estimate of how much money you would have to pay to get tickets for the same usage, using a consolidator, or course, buying just as far in advance as you would have to if you were using miles (often 6-10 months). Then do the math to get some idea of how much your miles are worth.
Now you can decide if the miles you would get are worth a fee to get them. If, for example, MCI offers me 10000 miles for using their long distance service for six months, but wants to charge me a $5 per month minimum (which I would never come close to exceeding), then I would be paying about 6 x 5 = $30 for those miles.; Let's see, that's $30/10000 miles = 0.3 cents per mile. Not a bad price for miles I will sell to the airlines for at least four times as much.
Since I value my miles at 1.6 cents each, I can consider that to be a 1.6% discount when I buy stuff that gets me one mile per dollar spent, say by using a credit card that uses miles, or shopping at internet sites that give me even more miles. This is useful for comparison shopping, of course, but it could be used as a negotiating point for trying for a cash discount, for example. (Note that businesses tell me that they pay 2.8% for every Visa or MasterCard transaction, and 3.2% for American Express transactions. So getting, say, 2% discount for cash is a win-win situation for both the business and me.)
And you can decide a cut-off value for which you would choose to pay cash for a ticket rather than use those valuable miles. For me, that averages about 1.6 cents per mile. But your decision might be different, depending on what kind of ticket you would buy with the miles.