From memory when I enquired 18 months ago it was around the £9000 mark. I'll give the girl on the phone her due, she was laughing too.
I thought my box was broken, displaying a lot of the symptoms commonly described on here. Businesses were quick to offer £4500 rebuilds. Instead I replaced a cracked pickup/filter and filled it with the correct amount of oil. Sorted.
Along with initial sealed for life servicing, incorrect oils, incorrect pickup filters, incorrectly done fill up procedures, failed valves, deteriorated wiring and failed sensors there is a myriad of reasons that could lead to a broken gearbox, or certainly the symptoms of one. A whole gearbox changeout is probably the fastest easiest way to solve it, not necessarily the cheapest.