In my opinion, the second one is not natural. Perhaps because of the fact that it's been the normal thing throughout history for prices to rise, by beginning your sentence as you do, it seems to suggest that they have failed to act normally. " The third sentence works because it focuses on the "unusual" aspect of current prices, rather than the "unchanging" aspect.
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AvangiUnless prices fall, I cannot live on my salary.Thanks, Avangi.