Objective function in Linear programming

I have done questions in linear programming which have objective function of Profit maximization and written as C = 3x + 4y where 3 is contribution of Variable X (Product A) and 4 is contribution of variable Y (Product B).
For example, organisation have function of cost minimization so it would be written as = 3x+4y. Here, 3 and 4 are marginal cost or total cost?