The Purdue University Corn & Soybean Field Guide published annually by the Purdue Diagnostic training Center contains a chart that shows it doesn't pay to replant on May 25 if you originally planted April 30 and have 20,000 healthy plants per acre left in the field. What you make up with a better stand you would more than lose due to later planting, based on long-term averages. And that doesn't even consider the cost of replanting.
However, if you experienced a serious issue with weather, like a storm after planting or if soils were just too wet and you didn't get good emergence, you may not have 20,000 healthy plants per acre. How long can corn populations go and it still doesn't pay to replant the stand?
Actually, the answer is 16,000 plants per acre in the April 30 stand. At that point you could expect 168 bushels per acre leaving the stand, and 174 bushels per acre if you replanted May 25. However, that assumes that 6 bushels per acre – or $30 per acre at $5 per bushel corn – would more than cover cost of replanting. That could depend upon the replant policy of your seed company. If you have to pay for the seed, even at half the cost, it may be a losing proposition to replant.
The scariest part is, what if you replant May 25 and don't get a perfect stand this time either? Suppose you only get 26,000 instead of 30,000 plants per acre. Then long-term yield potential would be 170 bushels per acre. Unless you're getting the seed free and won the diesel fuel in the CountryMark essay contest, that likely won't pay for itself.
The bottom line is that the stand needs to be very thin and the odds of getting a good stand by replanting corn this late in the game aren't great.