I use about $325 as a cut-off for use of a 25K domestic coach award ticket on most airlines. With the exception of last minute emergencies, I think paying $400 for a domestic coach ticket is a waste. To many destinations in the continental US, and certainly to larger cities and popular leisure destinations, some planning can usually get you tickets under $300. Obviously, better value can be had redeeming for international business and first class awards.

Note that I use the $325 hurdle on MOST airlines. As hindukid correctly points out, all airline miles are not created equal. One must take into account award availability on the different airlines, availability that may vary based on your current status with a particular airline. This leads to the conclusion that there is a difference, perhaps a substantial difference, in the value of miles across the various programs. In my view, ignoring these fundamental and real differences, and viewing every airline mile as worth 2 cents (or whatever value you assign them) is old FF thinking. For example, while I value AA miles at 1.3 cents each, I value CO miles at less than 1.0 cents, probably 0.8 or 0.9 cents each. The reason is that award availability on CO at standard levels, particularly for Hawaii and International Business First is just dreadful. It pretty much sucks for domestic awards too. Comparisons I have done between CO and AA standard award availability suggest that AA offers around 3 times as many FC award seats to Hawaii on each flight as CO does.