I would say, then, that the primary driver for a signal to expire is not for it to be common knowledge that it's a signal, but for it to be common knowledge that it's a cheap signal. Knowing which signals to pay attention to reduces to the question of accurately estimating how costly it is.

I agree. However, monetary cost is not the only such factor--the same logic applies to signals that require dedication to acquire and/or are difficult to acquire (learning to play a musical instrument, for instance). Note that the ivy league degree is difficult and requires dedication as well as being expensive, making it a very difficult-to-fake signal of status.