It is the estimated cost of one execution: the number of recipients the selected groups contain right now × the message parts × your per-message rate. The window warns that "Cost is based on current group members. Future sends may have a different number of recipients", because group membership (new customers, changed delivery days, opt-outs) is recalculated at each run. Balance After likewise shows your balance after one run only. Keep Auto Recharge on for long-running campaigns.