30 years ago, I took a g/f's Metro to have an oil leak fixed (drive shaft to gearbox). I asked how much, and the reply was £300. Whoa, I said, what parts do you have to replace. He said, 'It's not the parts, they're £3.50, but to get to them is a bugga'.
I often see people quoted for what seems like routine jobs, and they are astronomical. Clutches at 3k, electrical problems £1500, etc, etc. There is no thought on repair other than giving an excuse for a ton of work and huge bills at a garage.
Don't know if you got a garage quote for the work Bergs, but I'll bet you'd need some B&Ts afterwards to calm your nerves.
The only thing I can think of is that you're not supposed to repair the issue, but buy another gearbox??
Sympathies with you mate - sounds like a b*****d of a job.