In FunnelFlux you will notice multiple places where you can enter a cost for traffic.
Here I mean default cost for incoming traffic, not the update costs section, which lets you retroactively update your costs.
Aside from that update process, there are two ways to add cost data:
Using the flux_cost parameter in the URL
Using the traffic source overrides in a funnel's advanced settings
You can use cost updates in the data updates section, though this applies to historic data, not traffic as it enters.
Note that flux_cost is not appended to URLs unless you set a default cost, as otherwise its a redundant parameter to have in a URL.
Using the flux_cost paramater
There are several places where you can set a default flux_cost. You can do so at the traffic source level, and in a specific funnel (beside its name).
Note that these default cost settings do not set the cost of incoming traffic hits directly. Rather they just set the default flux_cost=xxx parameter that is added to the links that you generate for a funnel.
You can see this if you go to a funnel and click on any node > send traffic here.
As you change the traffic source you will see the default cost box change, and the flux_cost parameter will change in the generated URL accordingly.
In this case, the funnel-specific default cost setting, if present, will take priority over the traffic source default.
Using incoming traffic cost overrides
The second way to add cost to incoming hits is using the overrides available in the advanced settings of your funnel.
Note that this is different to the flux_cost parameter -- these overrides are the highest priority and will take priority over even flux_cost values that are being passed in the URLs. You can configure these like below:
These overrides exist so that you can update the cost of incoming traffic without having to make any changes to your URLs -- which is very important as it lets you avoid disrupting campaigns and ad delivery.
These overrides become active when you save and affect all future hits.
You can change them to update the cost applied to all new hits, and can delete them to revert back to using whatever is passed by flux_cost.
As with all traffic, you can then use the Update Costs section of FunnelFlux to update the cost data across all historic traffic.