If you take Steve Ballmer at his word, the only way Yahoo shareholders may be able to squeeze a bit more money out of Microsoft Corp is to pray for a stellar earnings report from the software company Thursday afternoon.

The Microsoft CEO has been talking tough all week from Morocco to Milan, saying he wouldn’t raise the company’s bid for Yahoo — now valued at $43.8 billion — even after the Web pioneer’s quarterly results came in a bit better than expected.

With 50 percent of the offer consisting of Microsoft shares (the rest is cash), Yahoo’s shareholders need a solid earnings report from Ballmer … or the value of the bid could fall further with Microsoft’s share price .

At Microsoft’s current stock price of $31.45, the deal values Yahoo at $30.45 per share (our math: [MSFT share price x share swap ratio of 0.9509] + $31 and divide the whole thing by 2). To get the bid back to $31 per share — the value when the offer was first made on Jan. 31 — Microsoft’s outlook needs to be strong enough to boost its shares to about $32.60, equivalent to a 3.7 percent rise.

But to get Yahoo shareholders a buck more to $32 per share, they’d need a blow-out quarterly report from Microsoft that would push its shares up 10.3 percent to $34.70.

The last time we remember Microsoft shares jumping that much was about six months ago on Oct. 25, when it reported fiscal first quarter earnings and lifted its full-year outlook on strong sales of Windows Vista and the “Halo 3″ video game.

Check out what Wall Street is expecting this quarter.