Apple (AAPL) added hi-def movie downloads to its iTunes store yesterday. The selection is pretty limited, but that'll improve over time.

But at $19.99, are they cheap enough to drive meaningful sales? In most cases, especially for big hits, they're 17% to 33% cheaper than hi-def Blu-ray discs, especially at Best Buy (BBY), and even at Amazon (AMZN), where prices tend to be lower. But in other cases, we're finding Blu-ray discs cheaper on Amazon than iTunes.

"Quantum of Solace" is $17 on DVD, $20 on iTunes, and $26 on Blu-ray.

"Twilight" is $18 on DVD, $20 on iTunes, and $24 on Blu-ray. ($25 on Blu-ray at Best Buy.)

"Transporter 3" is $17 on DVD, $20 on iTunes, and $26 on Blu-ray. ($30 on Blu-ray at Best Buy.)

"W." is $18 on DVD, $20 on iTunes, and $18 on Blu-ray. ($30 on Blu-ray at Best Buy.)

"Saw V" is $15 on DVD, $20 on iTunes, and $18 on Blu-ray. ($30 on Blu-ray at Best Buy.)



"Disaster Movie" is $20 on DVD, $20 on iTunes, and $18 on Blu-ray. ($30 on Blu-ray at Best Buy.)

In general, it looks like the instant hi-def download from iTunes is worth saving $10 instead of driving to Best-Buy for a Blu-ray disc. So that could boost Apple's sales. (Note to studios: You really need to start pricing Blu-ray discs cheaper.)

But each has its benefits and drawbacks. Blu-ray discs require a Blu-ray player (still pretty expensive) and either a trip to the store or waiting for the cheaper movie to ship from Amazon. But to watch HD iTunes movies on a TV, you have to hook up your computer or buy an Apple TV set-top box. Or you could watch them on a computer screen (which is admittedly how we're watching many more movies).

And hi-def iTunes downloads could face another challenge: Some Internet service providers -- most aggressively Time Warner Cable -- are experimenting with metered bandwidth and overage charges. So that 3.37 GB hi-def download could potentially cost another $3.37 to download, erasing some of the savings.