In a typical 24h period,
over 500,000 BTC is currently being transferred. 7200 BTC in 144 block generations are being produced. An average of 3500BTC is transferred per block. And about
25 BTC total per day are being paid in transaction fees. From this thread, we see we may want transactions fees sizeable enough to encourage inclusion of transactions in blocks, to make someone only interested in profit reconsider their wholesale omission of transactions.
A transaction fee is rarely required now, with so many old coins on the mature network. You can let 5 BTC sit in your wallet for a few days and you can spend them without fee. The current fees are only an anti-spam method, enforced only for small transfers of recently transferred coins, and will do little to eventually supplant mining rewards.
First, we may want to revert to the original Satoshi client's fees for low-priority transactions (and set a future block in the client where the old rules will again be enforced for relay and mining). It looks like the
change to 0.0005 BTC per KB fees (v0.3.23) was unenlightened. When the fee change was made, Bitcoin had a price trajectory that looked like this:
Secondly, I think it is time to also consider a percentage transaction fee.
A mandatory fee, to encourage inclusion of transactions in blocks beyond the anti-spam measures, is probably desireable, to the point where it becomes a not insignificant amount per block. 5 BTC per block or better may achieve this goal. To target 5BTC per block, by evaluating recent transaction history, this percentage would only be 0.14%. A percentage will not discourage microtransactions (developing nations, etc), and may be accompanied by the minimum fee, or only take effect for amounts above the minimum fee.
We know also that the mining reward will drop to 25 BTC by December. If we want to encourage blockchain difficulty to have the same resistance to attack it does now, knowing that Bitcoin isn't going to magically take over the world, we should strive to put transaction fees on a course to supplement and eventually replace coin generation, as we know will be required. Right now, if we consider 25BTC from fees to be the target for the next four years (replacing the 25BTC mining reward), we need to look at a rate similar to the percentage mentioned above.
Fees may reduce the amount of daily transfers, but this will also discourage the blockchain bloat we are currently experiencing (1GB to 1.5GB in four months). I've got a feeling much of that 500,000 BTC a day is moving around just because it
is free...