How to build a Cost function for process optimization ?

I am doing my research in process modelling and optimization. If anyone please suggest me how can I build the final cost function for maximizing energy and exergetic efficiency; and minimizing total cost of a complex refrigeration system or LNG process? I also find it difficult to correlate the decision variables (T, P, Refrigerant mass fraction) with the objective function. Since all the papers mention about the generalized equation of cost, energy and exergy and did not show the final form of their full cost function, I am getting it difficult to manage a way to build the final cost function.

Currently, I am using RSM and ANN to build a proxy model-based cost functions based on the output result of my simulation. But, what are the other ways to build a cost function?

[John D'Errico]

This really is not a question about MATLAB, but a problem about modeling for a specific class of problem. As such, you will be far better off if you ask it on a forum where it would be on-topic, and where you will find others who have the same interests. Only if you got very lucky would you find someone here to give you an intelligent answer, at least in a reasonable time frame. Most likely, the only answer you will get otherwise here will be one from a student 3 or 5 years from now, saying they are trying to solve the same problem, and wanting you to send them code to do their project.

[Rasel Ahmed]

Dear @john Thank you so much. Actually, i also asked this question in researchgate, but i did not get proper response. That's why i asked it here. Since, lot of people are now working on optimization, process optimization using matlab. Let's see if I am lucky enough to get any help !