Adding an extra Bedroom with Ensuite

My project details:

I am looking at adding an extra bedroom on the ground floor of my house. This bedroom will be built outside the existing house using one of the pre-existing walls and then connected to the main house via a doorway. This will make it appear to be part of the main house rather than an extension.

This project is being undertaken in North Western Sydney near Castle Hill. I have contacted a few builders and the quotations are ridiculous. At around $3000 sqm. I am looking at a cost of between $80K to $90K just to add another bedroom with ensuite. When you consider that my whole house cost me $209K to build, spending upwards of $80K sounds like highway robbery

Can anyone suggest ways of doing this cheaper? Does anyone know of any good builders who can do this for under $30,000 (this is my upper end of my budget)

All suggestions welcome