While adjustable transaction fees serve a number of purposes, my primary reason for promoting its implementation was as a protection against denial of service attacks and attempts to bloat our blockchain. Here is the question that is most important when selecting the optimal transaction fee:
If someone performed a denial of service attack against the network, would you be pleased with the revenue gained from transaction fees or would you feel the attacker took more value than they paid?
At 0.01 per KB, you can buy a block for $10. This means an attacker could dominate the network and delay many or most transactions for an entire day for around $14,400. A 24 hour attack would add up to 1.44 GB to the blockchain (which is currently around 700 MB). Would you be happy to have transactions disrupted for a day and have 1.44 GB of additional space used on your hard drive for $14,400 of revenue? If your answer is no, that means fees are too low.
At 0.05 per KB the cost of a 24 hour denial of service attack is $72,000. I would be quite pleased to see such an attack performed. If it happened repeatedly, we could use a small portion of the revenues to develop a method to trim the blockchain. Most of it could go to share buybacks and we would see the NSR price soar.
Our network is very open and provides access to untrusted parties by design. The simple truth is that we can’t prevent a denial of service attack, but we can decide how much it costs. We ought to charge enough for transactions that we would be pleased to see a denial of service attack. What is the minimum fee that would make you pleased to see a denial of service attack? At 0.01 per KB, I would be disappointed to see an attack. At 0.03 per KB, I’m not sure. At 0.05 per KB, I say bring it on!