There are many sources of discount Birkenstocks, however to find them you may need to look around a little in your area. Even if Birkenstocks are about the best value for your shoe money, they can be a little expensive, especially if you're outfitting the whole family.
There are three simple ways that you can find Birkenstocks at reduced prices. You can look at retailers of Birkenstocks for sales on last year's models, you can find overstock or liquidation style stores, or shop online and take advantage of reduced costs available online (often with rebates and sales that are only available online).
Another reasonably good place to check for reduced prices on Birkenstocks is through eBay. Many people sell Birkenstocks through online auctions, many at considerably reduced prices from retail, even on current year models. Since eBayers don't have to spend money on setting up their own retail presence, they are able to offer cheaper and discounted prices on Birkenstock sandals or any other footwear by Birkenstock. Just be sure to use the eBay that is closest to you to save on shipping costs.
Birkenstock sandals - if you've never had a pair you don't know what you're missing. You can wear them barefoot in summer or toss on a pair of wooly socks and boot around in your Birkenstock sandals all winter. Well at least that's what we do out here on the west coast.
For those of you "Back East" Birkenstocks and wooly socks may be a bit much, but you can always choose to wear clogs instead during the winter.
The sandals that Birkenstock makes are quite unique. They have a very sturdy cork sole that is really light and naturally water resistant so it holds up well in almost any conditions. The nice thing about the cork is that it slowly forms to your feet so that you get a completely custom fit that provides just the right support.
To be sure some people find that the arch in Birkenstocks does not fit their feet exactly right (I have had this problem), but by trying on slightly different sizes and adjusting the straps on the sandals you should be into a pair of comfy footwear in no time at all.
There are cheaper knockoff versions of Birkenstock footwear, but none of them can come close to matching the quality. The foot-bed is second to none, and the sandal straps are made of high quality softened leather so that the sandals are comfortable from the first day you walk in them. I've had pairs of Birkenstock sandals last as long as 7 years, no other cork-soled leather sandal I've ever owned has even come close.