For quick whiteboard diagrams, you can probably make do with any <$100 drawing tablet off Amazon. I personally use Wacom Intuos which is a bit pricer but have features for my digital art hobby (pen pressure and onboard buttons). Disclaimer: to make effective use of these tablets, you need to have good hand-eye coordination i.e. your eyes will be on the screen and won't be able to see where your pen is moving. It took me awhile to get used to using my tablet so YMMV.
If you have a compatible tablet, Apple/Android/Surface pens are $100-200. You'll have the advantage of being able to see where you are moving your pen.
If you don't have a compatible tablet, you can get a drawing tablet for ~$300 with onboard screen such as HUION KAMVAS.
You can read more about drawing tablets here: https://www.reddit.com/r/stylus/wiki/index