For just a little more than $250, I reckon the ZWO ASI120 MM / MC cameras are pretty hard to beat! (US$299 including express postage for the colour model, US$319 for the monochrome, and you'll get it in your hands within about 1 week of ordering.)
While they're primarily intended as planetary cameras, they are more sensitive than most webcams, and will do long exposures (up to 1,000 seconds, if your mount alignment is good enough!), so they can do basic DSOs as well, and you even get a wide-angle all-sky lens as well for doing skyscapes, meteor showers, satellite passes, etc.
http://www.zwoptical.com/Eng/Cameras/ASI120/index.asp