If you want to stay in an area with a nice beach and close to the wedding pick the Nusa Dua area of Bali, if a beach is not a necessity you can stay anywhere in south Bali and get a taxi which is cheap compared to Australia and Europe but the traffic can get busy in places.
Hotels can range from $40/night upwards depending on your budget, there are many resorts in the area also which offer a nice place to relax.
Good places to find deals are scoopon.com.au, deals.com.au, cudo.com.au, luxuryescapes.com.au