Basically it's a time consuming and likely to be messy job which no-one in their right minds would want to do...so if they have to, they will charge what they like.

Given the potential for drilling out a fixing. rethreading it, other potential issues, you could get it bodged for a lot less but bodged..

Are you sure the exhaust has to be removed to remove the sump?

From what I remember from oil changes on my daughters P reg 1Litre,the exhaust pipe is on the transmission end of the sump & it looks like there is room for the sump to come off. I dont remember if this car was a cat version or not.

If the pipe has to be removed might it be easier to remove the manifold from the block ?

Also remember that should you need a new front exhaust section that broken stud will have to be removed & the hole re tapped, so it may be better to choose your time & place to do it.