Definitely the decks I find at D.I. (Deseret Industries. Kinda like a Goodwill, but in Utah.)
I've found Cincinnati Rider Backs there for under a buck on numerous occasions. Just picked up a red jumbo index one last weekend.
As far as retail goes, I'd have to say the Charity Water decks. Awesome tuck case, cool back, phenomenal handling, and you can really support a great charity if you do street magic with them. Helps get the name out in addition to your $1 donation.
If 5 bucks is too expensive, you can never go wrong with Bee Diamond Backs.
When the Charity: Water deck came out, it was originally $10 and the entire amount went to the charity. I guess that was a limited-time offer of sorts.
You mention these and Bees, but the only way Bee decks get truly cheap is if you buy in bulk. You never really find a new pack of Bees a dollar each at a local pharmacy or big box store.
You have to look at shopbicyclecards.com (Wingra Direct, a retail company owned by USPC) and look in their closeouts section. This was where I snagged a pair of bricks of the Bicycle Big Gun deck for 99¢ each, and some others picked up the pink Harley Davidson "Butterfly" deck there as well. For now, there are no closeouts so the closeout section isn't even visible, but when there are, it's often unpopular but quality playing cards that are being dumped cheap just to empty the warehouse.
The beauty of the Big Gun deck was that it was only unpopular because of the idiotic pricing for what it was when it first came out. It was supposed to be the first reprint of the entire Victory series, the rarest Bicycle standard-issue decks because of their exceptionally short run, only a single printing in 1918. The originals were commemoratives to inspire people to support "The Great War" (World War I), but the war ended only a few months after the deck was created. Anyway... USPC decided that since custom decks were starting to become a big deal and people were paying big money for rare stuff, they'd create this artificially-rare deck, sell it only in pairs and charge a whopping $39.95 for them. Maybe, just maybe, if they'd done a stellar custom job on them, they would have been worth half that much, but they really ruined the deck by giving it a wicked-nice reproduction of the original deck's box, a faithful repro of the deck back, and 100% MODERN faces, just like any ordinary Bicycle Standards, right down to the jokers and the AoS code - most people didn't touch them at all until the closeout. If they'd been more intelligent about the pricing - maybe $4-5 a pack, $8-10 for the set - they wouldn't have found themselves marking it down to ninety-nine cents... Well, to be fair, they did price them to a lower, intermediate price in the initial stages of the closeout - I think it was $10 a pair - but by then the damage was done and people knew it was a pretty lame effort at a reproduction.