You might have noticed @Jason has placed a link on your previous thread to my article on How to build a pallet cubby. Please check it out and see if this is something you would like to build. Although I have used pallet timber you could certainly swap that out for timber purchased from Bunnings if you wanted a less rustic look.
It would be hard to give you a definitive cost on how much your 2.5m × 3m cubby would be. Perhaps you might like to share some sketches to give us a better idea of what you had in mind. You could have a look at our range of cubbies as I believe they are quite good value for the materials used in them and look fantastic.
We look forward to hearing more about your project and assisting you will the build.