What workshop is it? Are they using a genuine brand new Garrett turbo? Because I would not risk a rebuilt on a VNT turbo. If so, the price is reasonable, but I doubt that it is the problem, you should rather try someone else.
You say it is only occasionally? It could be MAF as well. In fact it could be a lot of things, but it seems like these guys just want to blindly start replacing things, starting with the most expensive parts first...