Modern envelopes, 1973 to now for POCON numbered postmarks, or pictorial postmarks, or even any other, depending on the value of the stamp and/or cancel to the collector (or accumulater !

), can be worth something also, perhaps $1 and up (or less

) for nice cancels (I'm a cancel colector.).
The $10 whale stamp, currently available (2014), is getting about $8 used or so by itself on
ebay now. With a nicely placed, readable, legible, cancel on envelope (cover) it would probably be $10 to $14.
Aesthetics (how things look) are always important.
If the stamp is in good shape (condition) then that is worth something also of course.
That common-to-you cancel may be rare to the world of marcophily (cancel collecting). The airmail label is collected, any other postal label is collected. Cancels on the back of envelope are important.