While I don't think compact cameras are ideal for shooting real estate I know there are Realtors out there trying to shoot acceptable photos with the absolute minimum cost so I won't be an equipment snob. Fact is, I used a Nikon CoolPix-995 with a wide-angle converter for several years and thought it was the best thing since sliced bread.
Two months ago at PMA-2009 Samsung announced three pretty amazing compact cameras:
All three of these cameras list for under $400 and have lenses that zoom down to 24mm effective which I believe wide enough for shooting interiors. Since these cameras were just announced in Feb they aren't shipping yet but they are close to being available.
Update: The problem these cameras have is it's difficult to use multiple flash or shoot bracketed shots to use in blending or HDR. Even though these cameras don't have any hot shoe or way to sync external flashes their built-in flash can be used to optically trigger one or more optical slave flashes in manual mode. You could pickup one or two older Nikon SB-26 flashes. The SB-26 optical slave has to be set on "delayed trigger" so the pre-flash (used by compact cameras to minimize red-eye) doesn't trigger the SB-26 prematurely. Becareful, not all flashes have this delayed trigger feature.
It's not very practical to shoot bracketed shots because you have to change the bracketing manually in between each shot and in doing that you risk moving the camera and tripod. I've done it this way before so it's not impossible, but you just can't do this consistently and get good results.
There you have it, probably the lowest price alternative for shooting interiors. Not the best but not impossible to use.