A lot of DVD sets can take years to ever get down that low though. Some never will (like the Star Trek ones). I would argue that The Simpsons sets are usually worth the $40 they launch at.
Actually, GoHastings.com ran the complete Next Gen collection for $99 awhile back.
Anywhoo, yeah, there are some sets that can be hard to find below $15 - and like I said, there are a few exceptions to that rule... but it's about smart shopping. $40 isn't bad for a Simpsons season set... until you see it for $13.99 five months later.
The only bad part about this plan is that you start to fall behind when you're not paying attention to the various sales and you realize you're missing about 14 sets and get the urge to buy them all at once.
But, on the bright side, I ended up paying an average of less than $9 a season for 11 different sets...
Simpsons 11 $15.92 (Go Hastings.com B2G1 Free - Used)
Simpsons 13 $13.99 (Amazon.com New)
Simpsons 20 $0 (GoHastings.com B2G1 Free - Used)
American Dad 2 $10 (eBay - used)
American Dad 3 $5 (Target 2-Pack New)
American Dad 4 $5 (Target 2-Pack New)
American Dad 5 $13.71 (GoHastings.com B2G1 Free - Used)
Family Guy 5 $12 (Walmart BF Sale)
Family Guy 6 $12 (Walmart BF Sale)
Family Guy 7 $12 (Walmart BF Sale)
Family Guy 8 $13.46 (Walmart.com with 10% off)
Cleveland Show 1 $13.99 (Amazon.com sale)
I don't want to think about how much that would have cost new.