I have a drafting chair + standup desk (modified IKEA Jerker). I can sit, or stand, total cost $60 (used). Both are solid and will probably outlast me.
The Jerker no longer being available is mourned by some http://www.ikeafans.com/home/modular-desks-ikeas-jerker-is-u...
It's $100 and it's almost perfect - the one thing I may do is add an angled footrest. But I'm not in a big rush.