The difference between 20x and 30x is not as big as you might think. Just take the fully-zoomed-out horizontal field of view which you can find in most spec pages, and divide it by the magnification number to calculate the actual field of view you would get.
For example lets assume you have a PTZ that starts with a 60 degree field of view. If you zoom to 10x, you get 60 / 10 = 6 degrees FOV. Zoom to 20x and you get 60 / 20 = 3 degrees. Zoom to 30x and you get 60 / 30 = 2 degrees. So you only gain half as much zoom going from 20x to 30x as you did going from 10x to 20x.
I guess what I'm getting at is that if you could spend the same money and get a 20x 2 megapixel PTZ or a 36x analog PTZ (standard definition) then you would be better off with the 20x 2MP hands down.