Agreed that you should advertise in such a way as to draw quilters because the general population won't be interested and you've put a lot of effort into sorting, measuring and bagging.
If I were yard saling, I'd think $5/gallon bag a good deal, but $15/4 would make me snag 4 if there was a big selection. Later in the day (after 12:30) I would expect a better bargain, say $2.50/bag or 5 for $10. But I would not tell anyone the prices will be lower after 12:30 or they'll take their chances and might not come back at all.