You said 1 buck is OK, not five. I said one relay and 10c parts. I guess that would not make for 5 bucks...
The other is
But don't you have a bulk cap somewhere? Just keep the relay in the same state until the bulk cap voltage is over. And if you use a low voltage coil WRT to the nominal bulk cap rail, then you have plenty brownout time before the relay decides to switch back.
Oh, and it'll never switch on 240 mains, so in not 3rd world countries we won't experience any failure at all.
Not yet, but I got some bit of interesting info I didn't think about, like the one (it's all about ergonomic) that use some "linearizing trick" to get a usable hand stroke. But I don't like the way they do it (1000s turns coil). Also the one you pointed me too was already preassembled. Just fit it in the box and voilà. That's not what I was after. But now I begin to feel I've the info I missed to design a good one, so I might go that path.
Of course work will get in the way, but...