But how much are they willing to pay to access music from iTunes via the cloud? A survey by marketing researcher NPD Group estimates that a base subscription rate of $10 a month would attract 13- to 15 million customers out of roughly 50 million iTunes users.

The report, which is based on 3,862 completed surveys from iTunes users age 13 and up, were asked their reactions to a music service offering free and unlimited streaming of content from the consumer’s own iTunes music library. They were also asked about several paid options for music subscriptions offering combinations of music streaming, music downloads and universal web access to their iTunes music library.

Between 7- and 8 million iTunes users in the U.S. told NPD they would have “strong interest” in one of the paid subscription options. In exchange for paying a monthly fee of $10—the minimum charge that NPD floated—they would want either streaming music or access to their personal music libraries on multiple devices. NPD estimates that there are 50 million iTunes users in the U.S. According to NPD’s music industry research, a model that offers iTunes users free access to their own music libraries would attract in the range of 13 to 15 million subscribers.

If the numbers of potential users of an Apple (NSDQ: AAPL) subscription service are true, that would translate to at least $1 billion in revenue in the first year. That’s roughly two-thirds of the revenue generated by the current iTunes pay-per-download model, Russ Crupnick, VP and senior entertainment analyst for NPD notes.

Of course, if users were to switch, it would almost surely cut into Apple’s music download sales. But Apple is in a better position than most to gamble, as iTunes represents 10 percent of its revenues, as Morgan Stanley internet analyst Mary Meeker recently estimated.

Apple is widely expected to be contemplating some sort of subscription music service, especially since it shut down song streamer Lala at the end of May, five months after it bought the site.

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