Where are Americans spending their rebate checks? Apparently Wal-Mart Stores, Inc. (NYSE: WMT) and Costco Wholesale Corporation (NASDAQ: COST). Both reported fantastic sales for May. Wal-Mart explicitly tied their success to the checks. "We're seeing some benefits from the stimulus checks," Wal-Mart Chief Executive Eduardo Castro-Wright said in a company press release. Wal-Mart same store sales were up 3.9% (4.4% if you include gas).

Costco sales were even better, up 5% from last year (7% if you count gas). Costco saw sales of $5.77 billion, up about $600 million. The theory is that people are turning to Wal-Mart and Costco because we're broke and because we're so freaked out by gas prices, we'd rather just drive to one big store. Nice theory, but Target Corporation (NYSE: TGT) is in the same business and saw sales drop 0.7%.

How many of us have spent our rebate checks at Wal-Mart and Costco? About 73% of Costco's stores are in the U.S. (The foreign stores got a big boost from the weak U.S. dollar). So, let's say very roughly 73% of Costco's $600 million sales increase was in the U.S. Or, Costco took in an extra $438 million thanks to the rebate checks. (Yeah, I know, the sales probably would've been up anyway, if only for inflation). At Wal-Mart domestic stores saw a $1.4 billion increase. Assuming we all spent our $600 in one place (just to make the math simpler), that would mean a little over 3 million of us (3,063,333) spent our rebated checks at Wal-Mart and Costco in May.