It’s being billed as the biggest day in entertainment history.

As “Halo 3” comes out today, the video game is expected to pull in more than $150 million in sales in 24 hours. By comparison, “Spider-Man 3” blitzed box-office records when it took in $151 million at theaters during its three-day opening weekend in May.

So are games really a bigger business than movies? Not quite.

Although the revenues from games have grown rapidly in the past few years, they are dwarfed by the sums of money generated by Hollywood blockbusters. Much has been made of the increasing similarities between games and movies, but actually their economics remain far apart.

“It’s apples and oranges,” said Jim Ward, who straddles both worlds as senior vice president of Lucasfilm and president of LucasArts, the company’s video-game division. “You have different price points, different distribution models, different business models. To me, the only thing they have in common is that they’re both entertainment experiences.”

Movie budgets, for example, regularly reach into the hundreds of millions of dollars, while game budgets rarely break $30 million. That’s a fraction of the amount studios spend on marketing alone.

Movies also generate far more revenue, largely because they have sales outlets other than theaters. Games have one shot – at retail. As a result, the movie business is projected to hit $84.3 billion globally this year, more than double the $37.5 billion forecasted for the games industry, according to PriceWaterhouseCoopers.

What makes the video-game business tantalizing is the potential profits. For Sony Corp., the estimated profit margin for “Spider-Man 3” is 46 percent, according to entertainment research firm SNL Kagan. Microsoft Corp., which publishes “Halo 3,” has the potential to see a profit margin of 80 percent or more for the game, according to analysts.

In terms of cost, “Spider-Man 3” weighed in at about $400 million to make and release. Producing and marketing the DVDs cost an additional $100 million, and Sony spent tens of millions on such expenses as overhead, profit participation and residual payments.

The total cost to Microsoft for “Halo 3”: a little more than $60 million.

When it comes to revenue, “Spider-Man 3” again is expected to smack down “Halo 3.” Altogether, Spidey’s third outing will bring in $1.366 billion in revenue for Sony.

“Halo 3” may have a total of $700 million in sales, said Colin Sebastian, an analyst with Lazard Capital Markets. Of that, Microsoft would get about 80 percent, or $560 million.

Although “Spider-Man 3” will generate an estimated $628 million in pretax profit for a 46 percent margin, according to SNL Kagan, “Halo 3” is looking at a profit of more than half a billion dollars and a margin of upward of 80 percent.

While blockbuster games such as “Halo 3” don’t always play in the same league as Hollywood hits, their relatively low costs mean they still can pack a powerful punch when it comes to profit.