I'd make a quick call to the Renault dealer service desk, ask them for a supply and fit price for the offending arm.

Cost of arm might well be about right, and it's usually better to fit a genuine part than aftermarket which sometimes arn't as good or last as long, the labour time seems long but having worked on Reanault cars many years ago may be right, some jobs were just horrible, especially replacement suspension parts where the OE parts were sometimes rivetted together so those rivets had to be either ground off if room or blown out with a cutting torch.

I based my answer on looking at a picture of a Master lower wishbone. It is similar to those fitted to my 9-3, ie two inner bushed mountings and an outer ball-jointed mount.

I did both my 9-3's wishbones on the floor in my garage in less than 3 1/2 hours.

Unless someone who knows Masters like the back of their hand cares to contradict me.

A quick gander at a couple of forums suggests it is actually quite easy to change the ball joint too. But that may depend on the year.

The original post says:

Suspension arm ball joint dust cover excessively deteriorated so that it no longer prevents ingress of dirt

I read that as the rubber dust cover needing replacement, is the garage assuming the arm itself will be so damagesd it needs replacement? A big cost difference between dust cover and complete arm.

They looked at the ball joint dust cover and the ball joint and were gobsmacked to see he had failed me on a tear in the ball joint that was not even 5mm in the rubber!

As far as I am aware that should be an MOT fail. Any split will allow dirt and water into the joint accelerating wear and leading to premature failure. Any MOT station that allows cars with defects like this should be investigated, who knows what other serious defects they ignore.

The fact that you have included details of a supposed friendly MOT station makes the thread look a bit like SPAM to me.

Normally if it looks like SPAM it is SPAM.