You should really stick to prime lenses if you're on the cheap, most good ones are under $500.
EF 28mm f/1.8 - $340 with discount at B&H
EF 35mm f/2.0 - $320
EF 50mm f/1.4 - $310 with discount at B&H
Sigma 30mm f/1.4 - $500
Sigma 50mm f/1.4 - $450
EF 100mm f/2.0 - $450
These are really bottom of the barrel prices, if you actually want to make the most of your camera, the only way to go cheaper would be to give up autofocus and use manual-focus lenses; but that would be a nightmare to use on the 400D. The 14mm is an exception because there is nothing that's out of focus with that lens, set it to around f/4 and focus it out to 2~4 meters using the scale, and everything from close up to infinity will be sharp. Focusing lenses wider than 20mm is often best done with the scale, so you'll want to read up on "hyperfocal distance", an article on the subject is available at the Cambridge site I linked you to earlier.
Personally, I'd go for the Sigma 30mm as it's a standard lens on the 400D, and it's quite better than the Canon 28mm. It can't be used on full-fame cameras, but that might not matter to you, as FF cameras still cost $2000 for the cheapest ones, unless you pick up a used 5D classic.
Do not think that a single focal length is in any way limiting, instead it often forces you not to be lazy and find creative ways to photograph things, instead of just zooming the lens. My 85mm stays on the camera 90% of the time, and I don't feel like there's anything I can't shoot with it, though it could just be related to my preferred subject matter (people) and when need be, I break out the zoom.
I only use wide angles 2% of the time, but that's just me, some people can't live without 'em. YMMV.