Pretty sure that this has been a semi regular price at retail for a few months, picked it up at Target at this price last year in fact, non-advertised price and not clearance. Seen it for 20 at Gamestop as well.
Retailers don't want to keep fighting games around, they are worse than music games when it comes to sales. That's why you see Mortal Kombat selling for 30 right now, the built in playerbase fighting fans already got theirs, and now retailers want to move their stock off shelves before the marketing dies on them and they become dust magnets. WB is happy to let retailers slash prices because they already got paid full price for the game (usually about $38-$42 wholesale) and the sales will help increase numbers for first month npds. After first month sales on fighters slump dramatically.
First Month of AE we will probably see up to a 20% promo in weekend flyers, but with the margin on a $40 game being nonexistent don't expect anything better until sales numbers crash.
To clarify on online modes, i believe all editions of Arcade will also be able to play against Super players when the disc and dlc release later this year, at least as I understood from the AE press release the game blogs posted. The presser said nothing about free costume unlocks, it's not impossible just improbable; 3D edition comes with all costumes for free... Let's all wish for it together and maybe tinkerbell will come to life.
Also people who buy on-disc dlc unlocks are chumps*
(**Digital Idiot has considered buying those damn SF costume packs multiple times XD)