Cheaper than Little Snitch[0], which has been the firewall of choice for privacy-conscious Mac OS X users for over a decade. I wonder how it compares. It's a shame that neither is open source. Also, I always wondered why there was nothing like this on Linux—I write firewall rules on my servers, but I just want to use my Linux desktop. sigh
[0] https://www.obdev.at/products/littlesnitch/index.html