As others have said, prices will depend on complexity and spec.
The only thing I can answer: you do not need planning permission for internal works in your house, and that includes converting an integral garage to a habitable room*. However, you will require 'building control approval' for the structural works (removing walls) and the drainage for the new toilet/shower and if the building is 'listed' you will need listed building consent.
*This assumes that your house was not originally granted planning permission subject to a condition removing 'permitted development' rights. This can sometimes be the case where car parking provision is so limited that the conversion of the garage would result in the loss of a car parking space and be unacceptable.
Thanks - good to know it has off street parking so loss of garage should not be an issue. I may have to rethink this and the hassle that’s goes with it!