The price per egg in a tray is $12/30 eggs = $<<12/30=0.4>>0.4/egg.
The equivalent of $0.4 in cents is $0.4 x 100 cents/dollar = <<0.4*100=40>>40 cents.
Hence, you can save 50 cents - 40 cents = <<50-40=10>>10 cents per egg if you buy a tray.
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