what did you have in mind for a budget?
If you are in the USA, The Canon direct website has refurbished lenses, and has a 15% off sale every few weeks.
The 17-40mm L can often be found used in the $500-600 range, its fairly decent for the price. For about $700 used, a 24-105mmL is also good.
As noted above, the 40mm f/2.8 is excellent and cheap!
At the lowest end I can recommend is the kit lens, 28-125mm IS found for $200-250 used. Its OK, but you might soon want more.
With a FF body like the 5D, most budget lenses will provide a good image with excellent resolution. That's one of the benefits of FF.