It would help if you specified the ones you already know of, so we can make sure our suggestions are cheaper.
The 5 cheapest models I see on NewEgg (I know it doesn't always have the best prices, and may not be available in your country, but it offers an advanced feature-based search) are:
Acer B243H AJbdr
NEC Display Solutions EA232WMi
However, if you just want the monitor mounted vertically, and don't need to frequently switch between horizontal and vertical orientation, there is an alternative which may be cheaper: find a monitor that has a square pattern of mounting holes (e.g. 75mm x 75mm or 100mm x 100mm, but not 200mm x 100mm), get a separate stand tall enough to support the vertical orientation of the monitor (or a wall mount or arm), and mount it vertically on that.