It isn't too expensive. Take your licence to the DVLA with 2 photos and you will fill in various forms in different areas and when you have completed the preliminaries you will go to the area where they take your photo and prepare the final licence. They issue you with a paper one first and then after a couple of months you can collect the plastic one. Make sure you also have your Ghanaian ID card as they might not give you one without it.
Sorry, I forgot that requirement! I was only in Ghana a few weeks when I got my licence 3 years ago. You could go to the office yourself as sometimes there is flexibility. The new ID card for foreigners is for everyone who is in Ghana for lnger than 90 days and the adverts say that it may be required when opening bank accounts and getting a drivers' licence. It will also be needed after 2nd September to renew resident's permit