How about a tall chair/stool? Seems cheap, though not as comfortable as a good chair. (But then again, maybe you don't want it to be too comfortable if you want to avoid sitting.)

Edit: see athenot's top-level comment.

If you don't mind making one yourself you can do it pretty cheap. I copied mine off http://www.codeovereasy.com/2013/09/diy-adjustable-desk-for-..., and it took just a few hours and $40.

Depends on where you look! < $500 feels very affordable. No experience with this desk in particular though.

http://www.ikea.com/us/en/catalog/products/S19022530/

I use the varidesk, I think it was $250 when I bought it. The quality definitely justifies the price, but I didn't find it too expensive to begin with especially since it was a write-off.

The adjustable Ikea Bekant sit/stand desk is "only" about three times the price of the non-adjustable Bekant desk. (I don't have one but have been tempted.)