Our agent charges for each part - so we paid him to do the skills assessment part, then once we have passed IELTS and go forward for visa there will be another payment - totallying £1800.
IELTS Test cost between £125 and £140 depending on where you go, then as you say cost of the Visa.
You could do it all yourselves but we went with an agent because our case was a bit difficult...............Good Luck