The World in Ancient Times set, a collection of books on Ancient history published by Oxford University Press, is on sale right now at Amazon.com and the publisher's site for just over $35!
This is a set that is wonderfully well written, has a volume of primary sources and usually costs over $200. I own the other set in the series, The Medieval and Early Modern World set (bought on sale as well although not nearly as good a sale) and can certainly vouch for it's quality. Even the husband, a history buff, has enjoyed reading through the set.
Use it as spine, as an extra resource or just put it on a book shelf and let the kids explore it but for gosh sakes, don't let this deal pass you by!