That's quite a vague question. I assume you want to stay with digital, but are you looking for a SLR or point & shoot? How long of exposure makes a big difference too. Digital has noise problems when performing long exposure especially for cameras under $500.

Here's a Nikon camera that shoots up to 5 minutes for around $300 but you will need some accessories like remote shutter cable.

Astromart has an excellent offer on one that has all the bells and whistles to go with it.

Best to do a lot of research and Sky & Telescope or Space.com should have some articles on cameras for astrophotgraphy (night imaging) which might help you determine what camera better fits your needs.

(I'm not Steve of digicams)