A number given a few years back was around $35k, but very few apps cost the average amount.  It typically takes a lot less or far more.  Many simple designs or poor quality apps have been made from templates, or quick turned by offshore shops, for a few hundred to a few thousand USD.  But some "simple" user feature additions can easily balloon the eventual cost by 10X or more.  And many top tier apps cost from a quarter million USD and on up.  If you have access to a recent Mac, you could put an app in the iOS App store yourself for $99/annum plus a very large number of hours following tutorials.