ThreadArt has the poly pre-wound bobbins on sale right now - 144 for $30 (The page says that there are 108 to the box, but there are actually 144 in the boxes that I've bought.)
http://www.threadart.com/shop/item.aspx?itemid=4055
This is the plastic-sided Type A bobbin for class 15 bobbin machines and it looks like they're selling for $25-40 per
80 bobbins elsewhere, so there's quite a difference in price. I love the thread in these bobbins, although I would like to find a matte poly or cotton, too.
They carry L and M sizes and in plastic, cardboard and sideless styles, but those aren't one sale at the moment. Still their prices are worth checking.