The cost can be tracked automatically by FunnelFlux only when the traffic source you use has a dynamic cost token.
PopAds, for example, has such a cost token called [BID]
When you setup your traffic source in FunnelFlux, just mention their dynamic cost token here:
Note: [BID] is only for PopAds - check with your traffic source's support to know what their dynamic cost token is.
Note: passing cost in URLs can be convenient, but is a) rarely available from traffic sources and b) can be dangerous. Consider that this reveals information to people who click on your ads, including competitors, thus can reveal details of your traffic source costs to people who may then use this information to outbid you. Pass at your own risk.