For the first 20,000 tracks, this service will be free. Instead of having to manually upload your gazillion songs to the cloud, Google will scan your library and instantaneously “upload” or well, match, the tracks in Google Play.
For comparison's sake, Amazon gives you 250 matched tracks and then charges $24.99 a year for this service (with the ability to store up to 250,000 songs) and Apple's iTunes Match also costs $24.99 a year for up to 25,000 songs.
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Google Music Will Have Scan and Match, Free for the First 20,000 Songs