I can second the Ikea adjustable leg desk solution for ~$125, and also the cardboard box solution. I've done both (the first at work, the 2nd at home).
However, I find after doing this for a few months, that I like to have the option of sitting sometimes during the day. Ideally I'd have a desk that can easily adjust from one to the other, but they're quite expensive. I'd also rather not have 2 horizontal surfaces attracting clutter (high desk + low desk) , so I tend to go back and forth with boxes. Its not ideal, as changing is tedious and the boxes clutter the room when not in use, but its good enough for now. There's probably a clever DIY replacement for the cardboard boxes but I haven't explored that yet.