Soldato
It's not a cheap fix at all and is something people really need to bear in mind when buying old BMW's but it's nothing like £6k to fix - putting a brand new factory transmission into an 11 year old car is foolish.
There are numerous used gearboxes and reconditioned gearboxes available on places like Ebay. I'd imagine you could get the car repaired with something like this for well under £2000.
Trouble is they're the sort of people who don't even have £2k to hand. Living beyond their means sadly.