The short answer, as others have mentioned, is that it really varies a lot by airline, by where you live, and by your personal travel pattern. The value of a given airline's (or hotel's or credit card's, etc.) points to one person will not be the same as the value of those same points to another person. Indeed, to some people, they may be completely worthless (e.g. on an airline in a region where you will not fly again before the points expire.)
Questions you need to answer
In order to determine the value to you of a given airline's points, you'll need to answer the following questions:
Which routes can these points be redeemed to fly that I actually intend to fly?
How much would I otherwise have paid for these tickets?
Do these points expire?
If the points do expire, do I fly this airline often enough to accrue enough points to redeem before the points expire?
If the points don't expire, do I fly this airline often enough to ever accrue enough points to redeem for a flight I plan to take?
How frequently and by how much has this airline historically devalued their points? In particular, how much will the value of the points change before I plan to redeem them?
How does this airline price award flight redemptions? Do they use a more or less constant dollar/point conversion (like Southwest,) a region-based award chart (like Singapore or Korean Air,) a distance-based award chart (like British Airways or Cathay Pacific,) or a dynamic point cost based on route demand (like Delta?) In most (but definitely not all) cases, the points can be redeemed for a higher dollar value on long-haul flights than for short-haul ones and in higher classes of service, rather than in economy.
Based on the above, which route(s) that I intend to fly on would yield the best point redemption value compared to the cost that I'd have otherwise paid for that route and class of service?
Determining the value to you
If the airline's points cannot be redeemed to fly on a route that you plan to fly on, then their points will probably be worth little or nothing to you (sometimes you could redeem them for things other than flights, but usually at bad values, well under 1 cent/point.)
If you will not earn enough of these points before they expire to redeem them, then they are worthless to you. If you will not ever earn enough of these points to redeem them, then they are worthless to you.
If you will earn enough of the points to redeem them for a flight, by the definition of value, the value of the points to you is the amount that you'd have otherwise been willing to pay for the route and class of service for which you choose to redeem the points.
In an answer to a question regarding whether airline loyalty programs can have value for casual (not business) travelers, I provided a couple of examples of point value to me. Note that this answer was written some time ago and both programs have devalued somewhat since then, so I wouldn't arrive at those same valuations today.
Some travel bloggers keep running lists of their valuations of several different airline, hotel, and credit card loyalty programs. For example, The Points Guy updates their valuations monthly, which also then allows for comparison of change over time. Note, though, that these valuations are how the guy who runs that site values the points based on his situation, not necessarily how you should value them based on your situation. In particular, if you never fly long-haul flights, you'll probably find his valuations to be more than what the points are actually worth to you.