I just got this in an email today - KKmoon Car Parking Radar System-Black Sales Online black - Tomtop. Rated 4.3 out of 5 with 600+ reviews. Certainly affordable at $8.40 + shipping from US warehouse. Also available from China and UK warehouses. I haven't tried it and limited to a 2 m detection range.
Very Low-cost backup sensor set
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Well that would be good for the front lower dam...
Too late for mine though...