Hello !
Did you try to fix the costs on your resources function and cost ?

Hello,
Yes, all my ressources have a cost.
To explain my problem (I am probably using the functionnality the wrong way)

I have created a fresh new project with tasks : screenshot "before.png". All is saved correctly
In order to monitor the global margin, I enter the price given to the client : screenshot "during.png" in the cost field
I save what I do and I have the screenshot "after.png". 
The cost field is now grayed and I cannot modify it anymore...
This example project is in a "qualified" state with the "under construction" flag on !
 

Check that you may not have activated the parameter that automatically calculates your validated cost
 

I checked the parameters, and all is set to No.

Hi !

In this case it is surely the consolidation of the validated work that you have put on always or on if set.
The fields become read-only and will automatically update from the lowest element of the WBS to go all the way up to the parent project.

Global parameters > planning > automation > consolidate validated work & cost

If you do not want to consolidate this data, set this parameter to "never", in this case the validated data is not uploaded automatically

Yes, correct !
You're my hero for today. Many thanks 🙂
Question solved !