Why abandon (should Add a document on stock consignment then also be abandoned because it is for now confusing)? The text of the proposal has to be improved so it makes everything clear. Maybe it’s a good improvement to let a native English speaker read the text and suggest changes / improvements.
I’m leaning toward ‘group’ because we are talking about a the cost price for a group of employees. So on company.employee you get a many2one field called cost_price_group and when calculating the cost price for an employee, it will look if the group is filled and take that cost price instead of the one on the employee itself. Also it should be made impossible to have both filled. So if one is filled the other will be readonly.
I would name the new model company.employee.cost_price_group or something like that. That model should also have the possibility to add new cost_prices by date like you now can do per employee.
Looking at the timesheet_cost module I would expect that this can be added directly into the module instead of creating a new one.