@holger80 has setup a site to monitor these costs which have been great to monitor for me as I am trying to claim as many accounts as I can but the RC costs have been fluctuating closely to the amount I have available. You can clearly see how the costs can move 10-20% in a 24 hour period. Maybe he can provide some guidance on the factors at play and how it could playbout in the future with more adoption, growth and Steem Price. On a positive note, the witnesses could control these costs fairly quickly and adjust according to what is best for the community and its protocol.