Making a power of attorney is relatively inexpensive – you can even do it yourself. You can download the forms from the internet, making sure you download the correct form from the correct websites and sign them in front of a witness (see more here).
If you need an enduring power of attorney, then you will need to seek assistance from your lawyer and there will likely be a cost to the service. The cost of legal services can vary according to the work involved, but generally, the cost of making an enduring power of attorney is going to be relatively inexpensive compared to the alternative.
If you do not have an enduring power of attorney, but you lose your mental capacity, anyone who wants to assist you with managing your legal and financial affairs would need to apply to the court. The cost of conducting legal proceedings through the court is going to be a relatively expensive exercise compared to what it would have cost you to make an enduring power of attorney in the first place.