Here’s how I’d imagine an exchange rate working.
Exchange for less: 1 Higher Traitstone for 1 lesser Traitstone. So exchanging a Runic for a Major would yield 1 Major.
Exchange for equal: 2-3 Equal Traitstones for 1 of another color. So exchanging 3 Runics would yield you 1 Runic of a different color.
Exchange for more: 5-10 Lesser Traitstone for 1 higher Traitstone. So exchanging 5 Green Runics & 5 Yellow Runics for 1 Arcane Light Traitstone.
From an economic standpoint this is the best way to incorporate such a system without offering to many resources to the player. Even if someone disagrees, this is the most likely outcome should an exchange ever come into fruition.