Lens for spare 20D

I was going to sell my 20D as it never gets used but it will only fetched $125 on eBay and the buyer fell through. So, I'm thinking I will keep it and just store in the car or use on family trips when lugging my 1D becomes a chore.

So, I'm trying to figure out which lens to get and don't want to spend more than $200 which limits my choices (I'm ok w/that). The three I'm bouncing between are the 40mm 2.8, 50mm 1.8, or the 18-55mm IS. I'm leaning towards the 40mm as I could use it on my 1D but any feedback would be appreciated.

The Canon 28-105 F2.5-4.5 II USM works well, is reasonably cheap, and covers you suggested focal lengths. A friend of mine used one very successfully on his 20D and later on his 1D3. It's not perfect - but it does a pretty good job - I have his lens now and am quite happy with it.
No it is not as good as my 24-105 but it is great on my EOS film cameras and comes along (because it is pretty light) with my wildlife gear - just in case.