It's a real expiration date; don't use past it unless you want to get burned.
We travel a lot, and we're very fair-skinned, so I buy sunblock year-round. After an incident about 2 years ago when I accidentally mixed up older for newer and got burned, I've started making it a point to prominantly date the bottles when I purchase them. I won't use them more than 9 months after purchase. (Also, be careful where you store your supply; don't leave it in a hot place for an extended period.)
The best deals I know of are at warehouse club stores; they tend to sell multi-packs at a markdown of around $2 per bottle. You can also sometimes get good deals from online stores, especially if you buy by the case and find a free shipping code.
Sprays don't go very far, so if you are trying to save, go with liquids if you'll actually use them. The recommendation for adults is one ounce per application, so the average 6 oz. can will last a family of 4 for about 4 hours of full sun, max. A 12 oz. bottle of liquid will get you a full 12 hour day, assuming that you reapply every 3 hours.
Personally, I like the spray because it is easy to apply, and that makes it worth the price for us. We will reapply properly and on schedule with sprays, but will sometimes procrastinate about getting sticky with the liquids, and that has lead to a few burns. For a week's trip to a Florida beach in summer, my famiy of 4 typically uses around 18 cans of spray. It's not cheap, but then, neither is melanoma.
For faces, we use wax sticks, and I really like the Ocean Potion dab-on for doing the parts in hair.