cost as % of transaction volume seems to hover around 2-3% [source] <– this *should* continue to decrease as the network grows

https://blockchain.info/charts/cost-per-transaction-percent?timespan=2year&showDataPoints=false&daysAverageString=1&show_header=true&scale=0&address=



others
http://bitbin.it/qAo2isuQ
http://bitbin.it/clQmt1KB