Received a quote from a garage in Glasgow.
Repaired in situ for £3270 plus vat
£180 plus vat without a car attached
They obviously don't want the hassle of having to take the box out of the car!
Are you sure he hasn't slipped an extra zero on by mistake?
[edit: The more I think about it, I would be asking if that was a typo.]
I guess the £180+VAT quoted is okay, if that's all in for the full bearing kit - works out around £100 labour?
It looks a straightforward box to split, and if you were really tight you could do the noisy input shaft bearings only at £25-£30, maybe some sundry sealant, and labour of course. Refitting would probably require a clutch & thrust bearing kit at £60-£70, and a couple of litres of Honda MTF or equivalent.
The problem with a secondhand gearbox is that while it may be okay now, you don't know how far along the road to failure it is. From what I have seen, a jazz gearbox for around £100+VAT would be cheap, there are plenty asking £200+. It's supply and demand, plenty of duff gearboxes about, and a healthy demand for good secondhand ones. Engines on the other hand, seem to go for peanuts.
As most of the labour is actually removing and replacing the box, the most sensible thing to me is to repair the original box. Most garages will remove the gearbox and send it to a local gearbox specialist, my MOT man does that with all gearbox faults. I think you would be looking at a few hours labour at each end, and he charges £30/hour inc. VAT.
On the subject of gearbox oil, I read earlier some discussion of the merits of more regular oil changes. It's a small manual gearbox on the Jazz, it contains only 1.5 litres of oil. That small a volume of oil will be kept very busy, and I would think that changing it more often would not do any harm at all.