I'd be inclined to negotiate the lease extension first, see if it's 'really' £50k.
It could be a lot more or a lot less.
You will also have a Surveyors bill and a Solicitors bill that add up to the wrong side of a grand (or two) on top.
It also depends on your age, and the age of the lease holders.
If you are a couple of decades younger than they are, then it's probably worth a punt.
If however they are a couple of decades younger than you, I'd be a lot less interested.
Also if this is a flat, look very closely at the Management fees.
We decided a number of years ago that we would only buy properties without management fees (and therefore usually freehold, but there are exceptions)
Remember property is a long term investment, where time is measured in decades not quarters.