$20 for the 42165 is a good price.
The 42165 is a plain "matt" screen with no focus aid. It's bright and clear, I've used it. Some people complain that the acute matte surface makes the last bit of critical focusing difficult (despite the brightness), I have not found that to be the case when comparing this to the 42204 (acute matte 2 version of this screen). There is a subtle difference in how the image comes into focus, but I'd be hard pressed to tell the difference if I did not have both side by side. However, this difference is the reason the new version is so much more $.
That said, I prefer a focus aid, and being someone that started out using micro-prism focus aids for action, I find it easier to focus with the old (darker) micro-prism screen than the new brighter matt screen. The one I use now is the split-image/micro-prism screen.
PS: make sure it really is a 42165, when people upgraded their screens, they would put the old screen in the new box. A true acute matte screen uses a prismatic surface in place of the random "ground" surface, if you hold it up in front of a point source of light, you will get a colourful prism like pattern.