Background
One of my hobbies for the last several years has been Android app development. My apps, while simple, have served some people well with a needed software tool. Recently, I was tasked with creating an app for a conference my wife coordinates every year. There was a twist though. For reasons unknown to me, lots of people seem to use iPhones rather than Android devices meaning the app would need to be built for both.
Enter Xamarin, or in this specific case Xamarin Forms. Xamarin forms seems capable of letting me build a good enough cross platform app for this purpose. However, there is a tax for iPhone development because you need to have a Mac to package and sign the final application, even if you develop in Xamarin on Windows. For this reason I needed the cheapest possible Mac that could run the latest version of Mac OS (currently High Sierra). At first I thought I would get a Mac mini, but the minis currently compatible with Sierra start in the $250 (US) range. I kept looking for something cheaper. It turns out you get an old MacBook for $100 less that would still run Sierra.
I therefore present to you fellow bargain hunters the cheapest Mac you can find that will run Sierra without any hacking: the Macbook A1342 family (Wikipedia Link).
UPDATE (9-19-2017): Great news! This Macbook can also run High Sierra (10.13). That means I (and you) should still be able to get some life out of this ancient but cheap little laptop.
Finding a Deal
My preferred way to find the cheapest absolute deal on used electronic equipment is Ebay. I have created for you what I think is the optimum search to find a good deal on one of these MacBooks on Ebay with the following search link:
-> Ebay Search Link
I hope you have good luck finding a good deal like I did. The best deal I found was a 2GB RAM model for $149 with free shipping. I then upgraded to 4GB RAM with some old DDR3 laptop RAM I had laying around.
The only drawback to these machines is that they are old and slow, but for a single purpose use such as mine, the price can’t be beat.