I think the determining factor for a good flaring tool comes down to 2 parts:
How much surface area is in contact with the tube to hold it in place and how accurate the die is placed.
Most $20 flare tools have a 1/2 thick clamp with multiple holes for tube sized. Well, take away a 1/4in for the flare bevel that leaves you with only about 1/4in to hold the tube in place.. not a lot to clamp and hold straight. The high end tools have at least 1/2-1 inch JUST for the line clamp.. much more clamping force.
Also the high end tools usually have a fixed mount for the dies, the cheap ones have a moveable horse shoe thing that needs to be lined up and can move.
Now just to find a $50 flare tool with those two features....
