Your 450D has a ‘crop sensor’ meaning that what lenses were wide on ‘full-frame’ film, are now less wide on your 450D as the edges get cut off by the smaller sensor. Consequently, a 24mm lens on your camera is actually a 24mm * 1.6 = 38.4mm lens. So to get an effective focal length of 24mm, you actually need a 15mm lens.
The next problem is that rectalinear wide angle lenses are very difficult to make, hence the expense. Also, as crop-sensors are a new concept, there aren’t many old lenses you could use with an adaptor, and certainly none which are autofocus.
My only suggestion is a gamble on a 2nd hand Sigma / Tokina / Tamron (all similar prices and quality), or that SamYang. I’ve read several good reviews about the SamYangs – MF shouldn’t be an issue for landscapes, but the lack of filter capability would be.