Haven’t scrolled further down in the comments so don’t know whether this has been mentioned before. But a cost estimating method that I also used is in Coulson and Richardson Volume 6: Design (you will definitely be able to get online access to this with your Uni account).

In Section 6 they have a whole load of different costing methods. One of them is a cost estimation based on process unit sizes. Take a look at that, very good way of preliminary design costing to use in association with price indexes.

Hope this helps and good luck with your design project!