Tervuren:
I think there was an observed, but not stastically correlated response. When combined with what Tobra pointed out about improved mortality in tight packed conditions, the process became de rigor.
As to your question, consumer reports seems to say that antibiotic free meat doesn't need to cost more.
Link:
http://consumersunion.org/wp-content/uploads/2012/06/CR_Meat_On_Drugs_Report_06-12.pdf
Quote:
"Whole Foods, offers nothing but meat and
poultry raised without antibiotics in its meat
department. Most other major chains offer
some such products. And the prices are not
prohibitive—a number of supermarkets are
offering chicken without antibiotics at $1.29 a
pound, for example, a price that is competitive
with all chicken prices nationally. Other studies
suggest that pork raised without antibiotics
should cost less than 5 cents a pound extra."