Well they are expensive, I think the cheapest new set I bought was like $9 once, but on average would expect to pay $11 or so.
I've bought most of mine at the thrift store, have found cases for like $5 each. I have also got a bunch for free, craigslist, freecycle and a friend once brought me 5 jelly jar cases he found in the alley next to the dumpster.
It can be an investment.
As for another place to check would be at Ace Hardware/ True Value stores. The more "old time" hardware stores almost always carry these items. I've bought a lot of my lids at Ace personally.