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After alternator, battery and equipment efficiency losses, you're probably looking at roughly 2:1 watts in vs watts out. Most aftermarket head units can put out ~50 watts peak, ~20 watts continuous. If we assume 100 watts constant draw at the crank to run the radio, that's about ~15% of 1HP, or maybe 1-2% fuel economy worst-case in most cars.
In my G1 I have a 640w amp and a sub on top of an aftermarket head unit, and generally speaking it disappears into background noise (lol) and I can still hit 90-100mpg blasting the radio. In theory I could be drawing up to 2HP at the crank for maybe a 20% loss in ecnomy, but I'd probably destroy my ears getting anywhere near that. Normal listening volume, even with an amplifier, there's no easily discernible economy loss.
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