gistfile1.txt

What’s the price per kilobyte of Ethereum transaction data?
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As of October 2016, it's about $0.023 USD per kilobyte (2.3 cents per kilobyte)

Step 1: Gas price:
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= 25gigawei per gas	(As of October 2016) https://etherscan.io/charts/gasprice

Ether unit conversion:
25 Gwei = 0.000000025 Ether http://ether.fund/tool/converter

Step 2: Price of ether in $
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=  $11.87 (As of October 10, 2016)  

So, price of gas in $
= $2.97e-7 per gas


Step 3: Gas costs per tx and byte of data
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See yellow paper http://gavwood.com/Paper.pdf
21000 gas “base” for a transaction
68 per (nonzero) byte of transaction data
20000 gas per “store” (256 bytes), i.e. 78 per byte

So, an approximation is 
= 75 gas per byte of data stored

Step 4: Cost per kilobyte:
$2.97e-7 * 75 * 1024 
= 2.3 cents per kilobyte