those are upwards or $50
No, they are not. Here is a Bluetooth plugin for Android and iOS for $10.
The rule goes like:
If you are one person, use that. You will save yourself time. If you are working for a company, you should make your own plugin.
is there a good tutorial for this that I can follow?
If you decide to make your own Bluetooth plugin, you must first learn Java. Make a simple Android Java Bluetooth program and test it on your Android device. If it works, then you can then simply convert it into a plugin by seperating the code into functions such as bluetoothInit(), bluetoothConnect(), bluetoothSend(), bluetoothReceive(), bluetoothDisconnect(), bluetoothClose(). Then compile it into a plugin (jar or aar) file. You can now call the functions from Unity C#.
Learn Android Bluetooth API.
Links on how to call Java functions from C# in Unity. I suggest you follow the first link.
http://www.what-could-possibly-go-wrong.com/creating-a-native-android-plugin-for-unity3d/
http://www.thegamecontriver.com/2015/04/android-plugin-unity-android-studio.html
https://www.thepolyglotdeveloper.com/2014/06/creating-an-android-java-plugin-for-unity3d/
EDIT:
For iOS, you need to learn Objective-C. Learn iOS Bluetooth API then make a plugin.
https://developer.apple.com/library/ios/documentation/NetworkingInternetWeb/Conceptual/CoreBluetooth_concepts/AboutCoreBluetooth/Introduction.html
http://code.tutsplus.com/tutorials/ios-7-sdk-core-bluetooth-practical-lesson--mobile-20741
