I have an N reg Polo, which needs a new gearbox, and I can tell you that the cost of a new gearbox is about £1000 from VW or £600 for a recon box from someone like VWSpares. Breakers prices are about £150-250.
Then you have fitting costs. Though the local Mr Clutch centre quoted me £700ish for g/b, clutch and fitting.
Though when the car only cost us £1000 you have wonder if it's worth it.
That said, it's a good fun car, better than the F*rd or V*uxhall equivalents.
*btw, those prices were for a manual gearbox, but autoboxes will be similar.