Found 13th Jul 2017
This is the cheapest height-adjustable monitor mount I could find (£36 inc delivery). The cheaper mounts just move in the horizontal plane whilst this also goes up and down using some kind of pneumatic spring. Of course it also tilts and allows rotation if that's what you're into. Max load is 6.5 Kg. I don't think this is a special offer so you've probably got plenty of time to think about whether you really want one.