Initial note: I'm currently using the INDI platform on Linux. Without developing a driver, this project won't work on ASCOM.
This is a project I actually finished many months ago. I wanted to extend my astrophoto setup with motor focuser and I haven't found any DIY focuser project which fullfils my requirements. So for these reasons I decided to create something from scratch:
DIY focuser is much cheaper than a manufactured one,
DIY is more fun,
I wanted something small and simple for astrophotography, without an external controller,
I wanted a focuser which can be connected directly to a computer (PC, Laptop or RaspberryPi), so USB seemed to be the best choice,
As always I wanted something aesthetic.
I was considering couple options, like Arduino+DRV or RaspberryPi+DRV. RPi is basically too big (and dosn't work on 12V out of the box). Arduino is a nice option (still no 12V), however more heavy in work as software needs to be written for both Arduino and PC. Then I found Pololu Tic controller.
It is relatively cheap, small and among others, it supports USB (no need for GPIOs or extra Arduino). Also what's important PC part of the software is open-source and well documented.
Pololu Tic T500 (source: official website): $19.95
NEMA-17 step motor (source: aliexpress): $7.11
DC barrel socket, wires, screws, mounting bracket, etc (guesstimate): $13.00