Minimum required specs of a Mac for iOS App development
Xcode 10 is available in the Mac App Store and includes SDKs for iOS 12, watchOS 5, macOS 10.14, and tvOS 12. Xcode 10 supports on-device debugging for iOS 8 and later, tvOS 9 and later, and watchOS 2 and later. Xcode 10 requires a Mac running macOS 10.13.6 or later.

Any 64-bit Mac with 8 GB of RAM and at least 200GB of disk space should do, if you don't care much about your time (compiles can be slow).

If I were scraping rock bottom, I would head over to https://www.apple.com/shop/refurbished/mac and wait for an 8 GB Mac Mini to show up. Check every morning around 8 AM PDT and again at 5 PM PDT. Be patient...which will prepare you for compiles that take 10 minutes each turn, but it'll work.

You'll still need a keyboard, mouse and monitor obviously, but you can get those at Best Buy refurbished. Also, consider Craigslist.org and any other site where you can buy used Macs.