What is the best way to assess...

Depends on lines and stats, but here's a basic outline:
Hidden Pot (unrevealed): increase price by 5%-7%
Rare 2 line: increase price by 3%-5%
Rare 3 line: increase price by 5%-6%
Epic 2 line: increase price by 70%-100%
Epic 3 line: increase price by 85%-120%
Unique 2 line: increase price by a hell of a lot. 500%+
Unique 3 line: 800%+ (almost always more than max mesos)

If it has 6%+ of a single stat (or allstats), apply an additional 160% to the price above for 6%, 230% for 9%, and 300% for 12%+