Most affordable sit-stand desk I've seen is from GeekDesk http://www.GeekDesk.com/

Still pretty pricey at $549 for just the frame. (Luckily its compatible with IKEA tops)

I wish IKEA would bring their adjustable legs back to the US, since their desks are so customizable.

IKEA has an adjustable desk system as part of the GALANT series. It doesn't show up in the US store, not sure if they stock it there. But in Sweden you can get, not super cheap, but better than a bad back and ass.

adjustable keyboard stand (desk attachment). sort of like those keyboard trays that slide out from underneath (which i hate), except the thing i'm thinking of lets you pull the keyboard to a decent range of positions in the XZ plane (if your monitor is the YZ plane and the surface of your desk is the XY plane), and thus can be used whether sitting or standing.

Use a podium. Very cheap.