I've got a Klipsch self-powered 2.1 ProMedia speaker system connected in my bedroom and den to Apple Express units (and an old iPod Touch in my bedroom so I don't have to keep my computer/server awake to play sleeping "masking noises" on repeat that plugs directly into the volume control part of the one speaker. For a small room like a bedroom or my office/den, they sound amazing! Cost? It varies, but it's usually around $120-150 and even Best Buy carries them (I think they've got through a few minor iterations since I bought mine. The express units enable them to be used as part of whole house audio (I can actually sync all the rooms in my house to iTunes music playback at the same time; that's 6 rooms worth).
I actually use a 3rd set (used to be used for video editing at a notebook/dock desk until I finished that work) for surround in my Carver system hidden behind my chair aimed at the side walls using my old Technics outboard Dolby Digital processor (doesn't get much use; the room is mostly for stereo playback, but I had the stuff already so I tried it and it sounded pretty good with DTS CDs, better than it ever should have with the surrounds, but they're that neutral).