-For most large scale ad servers, variable serve cost is less then $.01/CPM.

-The largest variable in this cost is normally bandwidth (of which there is essentially none for bidded impressions not purchased).

-The second largest variable cost is normally related to reporting (of which there is none or essentially none for bidded impressions not purchased).

-I believe pre-serve decisioning / bidding is a minority component of the sub-$.01/CPM variable serve cost for most ad servers (so think $.001/CPM range as a ballpark for a RTB function – and that’s without optimizing for a RTB environment, simply using more legacy ad server processes and their associated costs).

-This cost will be included into the smarter players algorithms, so that when it becomes cost-prohibitive, these undifferentiated / low success rate / less valuable impressions are filtered out beforehand instead of real-time decisioned on. A smart algorithm could handle automatically bucketing impressions into larger groups, and understanding which of these it is cost-effective to RTB on, and which are not cost-effective to RTB on and therefore should be excluded or fall under more broad non-real-time bid strategies.