No more than 50% of the cost difference between a new AT-VM510CB* and a VMN40ML replacement stylus.
So, that's (£95 - £165) / 2, meaning that your brother might have to pay someone up to 35 quid to take it off his hands.
Of course, there are lots of people who think that they can straighten the cantilever, and/or have some sort of misguided faith that it won't plough their records to death as is.
* Same improved body/generator as AT-VM540ML, the current equivalent of AT440MLb.
P.S. I notice that full retail for VMN40ML is £189.99 direct from A-T. £165 wouldn't be so bad a price for the equivalent of a new AT-VM540ML (£229.99 direct from A-T).