The Volvo dealer will probably charge you twice as much for this repair as a competent independent shop; and will mark the price of the alternator up way beyond its actual cost.
The way to get an accurate estimate of the cost of this job is to phone the Volvo dealer and ask them. Shop labor rates vary considerably in different parts of the country; as do dealership markups. This is why we cannot give accurate quotes for work done in different parts of the country. If you are concerned about the dealeship's honesty; phone some other Volvo dealerships and ask them for estimates.
And if you prefer to not pay through the nose for relatively common car repairs, I can recommend some good independent shops in your local area. Just click the "answer this question" button and ask for shop recommendations. Then click the "submit answer" button.