As with all other second fix aspects, this is an area that can break the bank. The choice of walls and floor coverings is a personal one, and costs here will vary widely.
When budgeting the build, the cost of a block will be fairly predictable but when it comes to the finishes, you will usually get an estimate, or PC Sum. This will be based on whether you chose a standard, mid range or high finish, and once you go and see what’s on the market you are likely to find that “standard” is below your expectations.
The choice of materials, and where you source them, will be the first cost factor, with the second being whether or not you can do the installation yourself – provided you have the time and skillset.