“This is amazing,” Glen Hansard exclaimed after he and his co-star Marketa Irglova strode onstage to pick up their Best Original Song Oscars for their haunting ballad, “Falling Slowly.” “What are we doing here? This is mad.”

Mad money, it turns out. Once, an indie musical from Ireland about a street musician and the girl he falls in love with, was made on a Scotch-tape-and-spit budget of around $150,000. Yet it managed to captivate audiences, become a darling of critics, snag an Oscar—and earn an astounding 10,000 percent return on its investment, in box office gross.

Now that the film is gone from theaters, the main beneficiaries of the film’s Oscar victory will be the soundtrack and the DVD, which landed in stores in December and at press time was the No. 12 bestselling on Amazon.com’s Movie & TV list. The film’s new Oscar and the pair’s memorable performance for the 32 million or so Academy Awards watchers is already moving more units and increasing profits for its label, Columbia/Sony BMG Records.

The soundtrack, which one industry insider says has been steadily selling 16,000 to 19,000 copies a week for two months, was downloaded from iTunes roughly 10,500 times between Oscar night and Monday, placing it at the top of iTunes’ CD list. At press time it was the top-selling CD on Amazon.com. The soundtracks from Juno and Enchanted (nominated for three songs), were No. 5 and No. 28, respectively, while Hansard and Irglova’s 2006 eponymous album, The Swell Season, was No. 15.

All that despite Once’s own David-and-Goliath subplot. The film opened in May 2007, just before the year’s eagerly anticipated CGI-laden blockbusters like Spider-Man 3, Shrek the Third, Harry Potter and the Order of the Phoenix, and Pirates of the Caribbean: At World’s End. Combined, these four films were shown on average in 4,285 theaters at their widest release. At its peak, Once played in a whopping 150.

With a global box office gross of $16.4 million, Once is far from the blockbuster of the year. Spider-Man 3 (worldwide box office: $890 million), Pirates of the Caribbean: At World’s End ($960 million), and Harry Potter and the Order of the Phoenix ($938 million). But the budgets for those films were also huge—nearly $250 million for Spider-Man alone.

“Studios usually do a bit of counterprogramming, but the success rate isn’t very high because that’s the season when you’re up against the big boys of summer,” says Jeff Bock, senior analyst for Exhibitor Relations, a Los Angeles-based firm that tallies box office totals. “You just don’t see this very often.”

Credit for the gamble goes to Fox Searchlight—a studio that turned low-budget indie films Juno and Napoleon Dynamite into box office gold (see slideshow)—for snagging the film’s North American rights for a reported $1 million at the 2007 Sundance Film Festival, where Once won a World Cinema Audience Award.

The film supplied something that summer event movies never do: a realistic love story and a soundtrack with indie-rock street cred. Largely a collection of aching ballads by Hansard and Irglova, the soundtrack is, perhaps, the best marketing tool a small film could have. “It’s an organic soundtrack and an emotional film,” says Bock. “In an age of cynicism, audiences gravitated toward that aspect of it.”

The movie was shot in 17 days in 2006, largely on the streets of Dublin. Director John Carney had initially asked Hansard, frontman for Irish rock band The Frames (Carney was bassist in the early ’90s), to write songs for the musical. When Irish actor Cillian Murphy, whose credits include Batman Begins and The Wind That Shakes the Barley, didn’t pan out for the lead, Hansard was asked to step in. Irglova, Hansard’s partner in the musical combo The Swell Season, was already on board.

After the film opened in the U.S., it grew slowly and steadily. Fox Searchlight had the best marketing that money can’t buy: word-of-mouth buzz and critical acclaim. While summer’s blockbusters came and went, Once stuck around theaters for an amazing 219 days—most of 2007—whereas Spider-Man and Pirates played for 112 and 133 days, respectively.

There’s one more life-imitating-art twist that may explain Once’s success in the middle of Hollywood’s blockbuster season; the onscreen love story carried over to real-life when actors/musicians Hansard, 37, and Irglova, 19, fell in love while shooting the film. “Capturing two people falling in love for real, that’s pretty powerful,” says Bock. Maybe even more powerful than big budgets.