you can get component hd or a vga hd cables for your 360.

any widescreen lcd 17" or larger (resolution 1280x720 or higher) can be used with the vga hd cable. to use the component hd cable, the monitor must have component inputs and be able to handle a 720p or 1080i (hdtv) signals.

bb had these 22" widescreens during their thanksgiving day sale for $199.
it has vga, dvi, and high-def component inputs so you could share it between your pc and xbox360 with either cable. (i got one and it looks great... maybe they'll restock soon.) bb currently has these 17" widescreens for $105.
this is a so-so monitor but for $105 you really can't complain.

this $400 samsung has vga and dvi so you can share it with the vga cable (assuming you have dvi on your pc).

here's a 20" benq for $275: