Last year I took time out to build a standing desk, it used adjustable legs to achieve the required height. The end result looked like this and cost about $200 or so.
At the time I was unable to find an affordable solution off the rack, however things have since changed and I have now discovered this.
The prices are quite reasonable and had this been around at the time I would have gone for this. The obviouse advantage is that the height adjustments can be made easily at any time, with my hand made model I have to clear the desk, upturn it and adjust each leg separately.
More information at VARIDESK Australia.