Futures are traded on margin, so that the P&L of any open position is realized on the posted margin. To maintain a constant exposure to the future, an expiring contract needs to be rolled into a new contract. I have read that the cost of doing this (using a calendar spread for example) is just the difference between the prices of the two contracts. I don't see why this is the case - all the P&L on the original position would have been realized at the time of rolling, so why would we need to pay or gain the difference between the two contracts?
Additionally, am I correct in understanding that a backadjusted price series does not take into account the cost of rolling? I.e. to get a proper account of a trading strategy backtested on this data, we would need to add in the cost of rolling (plus other costs).