You might want to look into used racecar seats. As I understand it, certain parts have to be replaced after a certain period of time (helmets and I believe seats). If you're okay with buying used, you can just peruse for sale sections for performance cars/racing forums. You might be able to find a pretty good deal.
In my experience, 15-20 lbs is the lowest weight you're going to see for seats in a reasonable price range. And depending on how heavy your stock seats were, you might only be seeing about a 10-15 lbs savings. Some seats have built-in airbags, so they might be even heavier.
Regardless, weight savings (ironically enough) tends to be the most costly of all modifications one can do to a car.
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