Spartacus: Complete Series (Blu/UV): $70 (lowest was $45 in December 2014...urgh, and I missed it).
Twin Peaks: The Complete Series (aka Entire Mystery) (Blu): $75 (LOWEST PRICE EVER!)
Firefly: Complete Series (Blu): $12.99 (Again, hit the lowest recorded price)
True Blood: Complete Series (Blu): $169.99 (Again, lowest price but not the best) (Also DVD is $140)
Charmed: Complete Series (DVD): $45 (90s nostalgia for a great price)
Buffy: Complete Series (DVD): $65 (More 90s nostalgia for a great price...may not be the lowest price)
Columbo: Complete Series (DVD): $80 (a particular kind of investigator...if you love the likes of Agatha Christie's Poirot, Monk...Columbo is another staple series that falls into that vein, and $80 for a classic series may not be too much to ask for)
Some other noteworthy mentions:
Psych: The Complete Series on DVD $90 (I love the show, but I'll wait for it to hit less than this)
There's also Boy Meets World and Sex & The City for Complete Series, as well...and that's it.