Paul,
The best performing Canon wideangle I've seen for astrophotography if the 85 f1.2L, but sorry its not a budget lens!
Back to the budget (under $1000 - if purchased OS from BH Photo or similiar) you've got plenty of options:
Thinking of astrophotography as the main use I'd consider:
Peleng 8mm Fisheye - Cheap, manual focus, but have seen some impressive results! Not a true fisheye unless used with 35mm format.
Canon 17-40 L - possibly your best bet at the true wide-angle (at 17mm).
Sigma 17-70 - Cheaper alternative to 17-40, with more range. Sharpness a little lacking at extreme ends wide open perhaps and it doesn't cover full frame.
Canon 28 f2.8 - I had this one, sold to mike. Very useable wide open.
Sigma 30 1.4 - I've seen some stunning results with this lens. Apparently AF can be a bit touchy on these lenses. Not a full frame lens.
Canon 50 1.4 - Perhaps even sharper than 50 1.8 and with USM and distance scale.
Canon 60 2.8 Macro - Should be an excellent astro lens - but haven't seen any results yet.
Canon 85 1.8 and 100 2 - Similiar designs and both sharp lenses, similiar in performance to 50 1.8. I've heard the 100 2 is margninally better for astrophotography.
Canon 100 2.8 - I have this and it is a great astro lens, wonderful for portraits and macros (I've been considering selling this lens due to lack of use, but just can't bring myself to do it!). Here is an image made with it wide open
http://www.pbase.com/terrylovejoy/image/36529489
Canon 135 2 - Everyone I know who has used this raves about its performance. Excellent performance for astrophotography even wide open. BTW you will get one for $1200-1300 if you look around.
Canon 200 2.8 - Not really a wide angle, but highly recommended!
You can see some nice work done with the 135 2 and 200 2.8 here:
http://astrosurf.com/buil/gallery/roph/img.htm
Terry