Four shapes, and between them they explain most of what any name is worth. Length first, then whether the string is a word anybody already knows.
The scarcest shape there is. 46,656 possible strings across letters and digits, and 17,576 using letters alone.
Thirty-six times the supply of three, which is exactly why they cost a fraction as much.
Numeric handles read the same in every language, which is why they trade especially well outside English-speaking markets.
A name somebody would type without being told to. Category ownership is the single largest premium in the whole market.