Programming, design, and art are your three necessary costs. Some might question whether design is completely necessary, preferring the scribbles-on-napkin or fast-iteration approach. Though there is some merit to this when dealing with simple apps, any well-developed, complex interactive app requires a great amount of design planning and forethought, or else you're taking huge risks with scope, budget, and time.
Audio isn't always needed (for utility or business apps), and, fortunately, when it is, royalty-free libraries of sounds and songs are available on the Internet (search for “royalty free sounds” for a decent list). Reasons against this approach might be that the quality isn't always there, legal restrictions sometimes exist for particular sounds/songs, and it can be difficult/time-consuming to wade through all the muck on your way to finding a gem.
With that being said, some amazing, under-utilized audio engineers and composers are available for hire, and these people will create all your sounds from scratch, or know how to find them quickly and modify them to fit your needs.
Expect (non-VO) audio to range from $500 or so for a simple game with a few core sounds, to $5,000 or more for a full game. When VO work is required, audio costs can skyrocket. Sometimes it can be more than $10,000 for good talent per actor, again depending on what's needed and the type of talent you need.
Special effects are considered ...