Yeah, those Nyl-glo flags that sell for $30-40 in WM and other chandleries are one big rip off.
We have taken the opposite approach to quality - we buy a several screen
printed flags for $3 each. I don't know if they are the polyester ones you speak of, although they definitely are not double layer. However, they last 4-8 months, depending on the wind
. 4 months in constant trade
winds, and 8 months in normal variable winds.
We have never had a $30 Nyl-glo flag last more than 6 months in the trade