You live and you learn in this game.
$300 for the miles you got back via re-booking is a palatable learning experience in my opinion.
I think we have all "overpaid" a time or two for an award when there was better inventory/routing/planes to be had. But does that negate the great deals and savings we usually find? Not by a long shot.
But do realize that sometimes, if you need to get somewhere, that you might just have to pay the big bucks (or big miles in this case) to make sure you get a ticket. The primo inventory may never appear in time to get it ticketed.