Yes, it should be pretty cheap and simple to build a player device that uses Secure Digital (SD) memory cards. With a little care and a lot of spare time (Hah!), I should be able to do that now by buying a flash MP3 player and rebuilding it in a car radio chassis. Maybe you could, too.
However, as soon as you bring a USB port into it, the complexity goes up huge amounts. MP3 chips that have SD interfaces usually don't have USB host interfaces. Embedded chips that have USB host interfaces are usually full-featured CPUs with extra expense to use, compared with the simpler chips.
Also, music collections tend not to be on SD cards. Apple and Creative and Microsoft and so on want people to put their music onto devices that attach via USB. Apple and Microsoft also hide where the songs are on the music player, so that makes the USB approach even more difficult.
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