If you require tickets for a general audience on Wednesdays, they are free.
You need to order by fax direct with Vatican to be picked up from there.
If you would prefer to have them delivered to your hotel, you can pay 10 euros with Maximus Tours of Rome. I have used them previously and the tickets usually arrive day or two before audience.
I have seen another company called Vatican tours Online and they are currently charging 20 euros per ticket for delivery.