SWA does not discount the "infant fare" off of the same fare the adults in the party are paying. SWA discounts it off the full-price refundable fare, which usually ends up making it much more expensive than the price paid by the adults. You get better RR credit for buying online, anyway, so on SWA it usually does not end up being wise to buy the "infant fare." SWA is US-domestic only, of course.
Most int'l flights I've taken on US carriers have discounted children's fares of about 20-30% off whatever the adults in the party pay. This fare would be the same for infants if the baby has a separate seat. On int'l flights, the "infant fare" that costs only 10% of the fare the adults pay is actually the fee you must pay to get your child on the passenger manifest and carry him as a lap baby, lap babies are not completely free on int'l flights.