20mm cheap FC for experiment

Hi, I’m looking for a cheap 20*20mm flight controller to experiment with ardupilot on a racing drone with gps that I have. Any suggestions? Thanks in advance.

What do you consider cheap? Omnibus F4 Nano was the cheapest. Not sure if you will find one other than direct from Airbot and there are a few versions. Kakute H7-Mini. Matek H743-mini would be the best choice but also the most expensive.

Before asking about other cheap FC’s you might find check here 1st for supported boards: