That would be Amazing Spider-Man #1, which is rumored to have sold over 700,000 copies.
Marvel made hand over fist with ASM #1. Assuming Marvel takes in 40% of the cover price, that means they earned $1,680,000 off ASM #1. If the issue had been $4 rather than 6, they would have made just $1,120,000. That is a difference of $560,000! To put that in perspective, each issue of Superior Spider-Man sold an average of 76,000 copies over the last three months. Assuming the same 40% cut, that means each issue of Superior made Marvel $121,000, which means that two dollar price hike was worth 4.6 issues of Superior Spider-Man!
Thinking about it in this way, the way DC has been upping the price tag of a few issues of Batman by one dollar is actually hugely significant to DC's bottom line. If a normal issue of Batman brings in $184,000, then a five dollar price tag will rake in $230,000. That is a difference of $46,000, or one quarter of a single issue of Batman. To put that number in perspective, Supergirl earned DC $27,000. So that one dollar price hike on a single Batman issue is worth 1.7 issues of Supergirl (or, more interestingly, 3.8 Batwing issues).
April 2014 Comic Book Sales Figures
Batman Eternal 1-4 combined sales of 316,392 units + the 108,998 of Batman 30 = $$$$
Diamond's estimate of 532k for Amazing Spider-Man shows how incomplete a monthly snapshot of sales are. The books sold on the last week don't get reorders until the next month.
Batman had some really low sales there. 108k? That's down quite a bit from a month ago (which was around 113-115k). Maybe Batman's sales were being cannibalized a bit by Eternal this month.
Sinestro seems to have pulled in solid numbers, even if it was not a home run. 46k is very respectable, especially given the rest of the family's numbers.
The guy who owns my local comic shop said Ultimate FF was the first Marvel Now #1 to not sell a single copy in its first week. That does not bode well for the title.
Just more of Marvel editorial's neglect towards the Ultimates lines. They were actually making a great turnaround with Hickman on Ultimates, but they took him off there to do Avengers and replaced him with Humphries of all writers. I think the results on Avengers and Ultimates really speaks for itself on the foolishness of that move.
Still UFF had a decent run with Bendis, Ellis, Millar and Carey.