If the transmission is failing due to bearing issues then £600+ sounds like a good price for a recon transmission. I used to work for Nissan UK and did hundreds of transmission rebuilds as Nissan had problems with them (mainy diff bearings) in the Nissan T12 Bluebird era. I think back then a tranny rebuild was around £500 and a recon would have been similar.
As for the cause.. A gearbox is a pretty robust piece of kit, and unless it has been poorly treated.. e.g, not serviced/adjusted/lubricated properly, or the clutch has been dropped harshly over and over with many rough gear changes thrown in, it should last the lifetime of the vehicle. Either that or their is an inherent problem with the likes of bearing quality or assembly proceedures creating an issue.
Normally bearing problems will just be innitially noticed as a slight whine or rumble, but over time it may get worse untill eventually you'll have a heavier whine or rumbling sound depending on which bearings are failing.
It should not be ignored though as bearings can and do completely fail, and if this happens it can result in a breakdown at best, or at worst having the gearbox literally turn its insides into a scrap bin.