This is a chart outlining the Sapphire Use compared to offense->defense provided when purchasing units in the Black market.

This chart does NOT take into consideration things such as (Lost Art Benefits/Sapphire Discounts) This is purely stock offense defense stats

Also keep in mind that all units have there specific benefit in a specific field (best example is the thief)

This chart is in order of highest value > lowest value, based on sapphires required per defense/offense/spying point

Offense unit comparison

Dragons -> 10.05 Offense Per sapphire

Wyvern -> 9.41 Offense Per sapphire

Necro/Warlock ->7.82 Offense Per sapphire

Renegade -> 7.5 Offense Per sapphire

Knight/Great Lord -> 7.05 Offense Per sapphire

Chimera -> 7.03 Offense Per sapphire

Pathfinder -> 7.03 Offense Per sapphire

Reaver -> 6.54 Offense Per sapphire

Thief -> 3.84 Offense Per sapphire

Defensive unit comparison

Undead Dragon -> 8.011 Defense Per sapphire

Griffin -> 7.07 Defense Per sapphire

Demon -> 7.03 Defense Per sapphire  

Golem 7.01 Defense Per sapphire

Barb/nomad -> 6.81 Defense Per sapphire

Huntress -> 3.75 Defense Per sapphire

Bear-rider/Arbalester/Ballista/warden -> 2.7 Defense Per sapphire

Spying unit comparison

Succubus -> 1.53 Spying Per sapphire

Silent One -> 1.25 Spying Per sapphire

Assassin -> 0.78 Spying Per sapphire