The value of flying depends on the creature that has it. Eagle Wings overestimates the value of the flying trait in order to avoid creating broken combos (as does every other card that grants a trait to a creature); assuming Eagle Wings is balanced correctly, the value of flying on a non-mage creature is between 1 and 6 mana, depending on the creature.
Best example:
The value of flying on a level 1 animal with 5HP, 0 armor, and fast is about 1 mana ([mwcard=MW1C37]Thunderift Falcon[/mwcard] vs. [mwcard=MW1C04]Bitterwood Fox[/mwcard])
Unfortunately, most other flying creatures have unique abilities that make it difficult to compare them to similar non-flying creatures. Looking at another rough comparison:
[mwcard=MW1C38]Timber Wolf[/mwcard] (9 mana) vs. [mwcard=MW1C19]Gray Angel[/mwcard] (12 mana):
Both have the same attack, life, and armor, making them roughly comparable. Redeeming Sacrifice does complicate things a bit, but if we assume that it is roughly equal to the value of the animal subtype on the wolf, flying here is worth 2-3 mana.
If I had to make give a rough estimate of the value of the flying trait, based on the woefully inadequate sample above I would say that it is about equal to the level of the creature that has it (in Adramelech's case, 6). Obviously not an exact science, but a decent ballpark estimate; game conditions will affect this as well (in particular, whether the other mage came prepared to face flyers).