- Low end: does the basics, slowly, but gets the job done: maybe free to maybe $300
- Midrange: does the basics, bit nicer looking, more reliable, and faster, maybe a few more features: $300 to $600
- High end: super fast, all the features, robust and long lasting $600+
Manufacturers are making faster, more capable phones, trying to improve state of the art, speed, brand image. Many people just want to make a few calls, send a few messages, check their email, and maybe play a game when they're bored. You don't need a supercomputer for that.
It's subjective but mid tier pretty much covers the Samsung "a" range up to the iPhone SE. It usually has a few contenders from last year's flagships. Anything from $270-$800
Above that you are in flagship territory. Whatever that means to you.