A long time ago, my dad did an informal study on some South Devon weaning calves. He fed them an average of 10# per head per day of Purina Preconditioning and Receiving Chow. They gained an average of 3# per head per day. With rounded numbers (for simplicity), at today's prices, that would be a cost of $1.50 per head per day ($7.50 per 50# bag or 15 cents per pound of PreCon). Assuming the calves brought $1 per pound (again for simplicity) that would be an average daily gain of $3 per head. Which translates to a daily net gain of $1.50 per head. With PreCon there are no necessary hay costs, so if you are going to use a standard type creep chow (which will be cheaper) you will need to figure in hay costs as well. And to really do it right you would need to compare growth, condition, etc. of the creep fed calves to some similar calves on grass only (or still on their mama or whatever situation you want to see if creep feeding will improve).
Also, if you make it known that your calves have been creeped, some buyers like that because after transport they will go right to eating, rather than bawling for their mama for a couple of days.