The focal length you want depends on what you want to do with it, and also if you want to use it on a crop camera, or full frame.
I think that a very common purpose for a tilt shift lens is architecture, and for this I would suggest the wider lens of the two. The tilt shift possibilities of the two lenses are almost the same, but due to the different focal lengths, the final images will of course be different. So you really have to think about what you want to photograph, and which focal length is best suited for that. Just as you would do for any other prime.
Tilt-shift properties (source):
- TS-E 45mm f2.8 (1991) – Maximum Shift = 11mm. Maximum Tilt = 8
- TS-E 24mm f3.5L II (2009) – Maximum Shift = 12mm. Maximum
Tilt = 8.5 degrees.
To comment on the price: in general wide angle lenses are more difficult and therefore expensive to produce. A very simple example is the canon 24mm 1.4 vs the 50mm 1.4, the price difference between these two is huge. And yes, I know the 50mm is not an L lens, and the buid quality is less, but even the 24mm f2.8 is more expensive already. Furthermore the design of the 24mm ts-e is a lot newer than for the 45mm, and this has probably also an impact on the price.