It depends on many factors, current sales, how complex it is, competition, time to develop, upgrade plans, artwork used, popularity, etc...
The 'average' price would be pretty meaningless because you have a ton of simple apps and much less higher end apps. A calculator app would be worth about $1 vs a high end game popular might be worth $200K+
Also, if an app is on the upswing it should be worth more, some popular apps just got kicked off the appstore, that would kill their value.