If you have access to some kind of software instrument (e.g., Garage Band for Mac), a keyboard controller is the way to go if you want something that plays like a piano in that price range. I've used this before, and it's really hard to beat for the price:
Buy M-Audio Keystation 88es USB MIDI Keyboard Controller | Keyboard Controllers | Musician's Friend
If you're looking for something self-contained (onboard speakers & synthesis) I think Yamaha are pretty decent, and you can stay within your price range easily. The main drawback in going that route though, is that you're going to have non-weighted keys and likely won't find anything over 61 keys. I haven't played around with around with the modern ones, but I would hope they allow you to modify the tonal range on a 61 key layout (this way, if you want to use the lower couple of octaves that are left off of the hardware, you'll have the option). It wouldn't shock me if they left that out of the entry-level boards to entice you to upgrade to a more expensive model though.
edit-
sounds more like you're in the market for a basic digital piano. this looks like the closest offering to what you're looking for, although it is above your stated price range:
Buy Yamaha NP-30 Digital Keyboard | Stage Digital Pianos | Musician's Friend