Will digital content ever be cheaper than DVD's?
No, because forum posters always compare the iTunes price which is set by the studio, to the lowest priced physical copy they can find, which is set by the retailer with the most excess stock or the least interest in profit.
See the above Fight Club example, where the DVD priced is claimed to be $4.75 because that's the Amazon price, but the list price is actually $19.98.