Maybe you should look at moped or motorcycle signals? If you find a good solution please do share it, I briefly looked and was surprised that most of the options looked like junk. I would guess anything designed for the bike market that isn't junk would cost several times your budget.
Ever see blinking lights on a bicycle from behind? I wonder if you can count on drivers to discern a turn signal from a standard bicycle blinker. Most of the solutions I've seen have the L+R blinkers too close together to count on a driver recognizing what you are signaling.
I don't disagree with your reservations about a helmet, but I do think they have much better visibility for drivers than low mounted lights on a bike. This will be about $120 shipped for the MIPs version. If it turns out to be a decent helmet, then the lights are like a free bonus.
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