Table of skillbook droprate and where to get them:
http://pw.solandri.com/skills.html#cWZa*Token cost: 18k each (I take average price)
*30 Advanced Mystical Pages = 450 tokens
*Each skillbook from Advanced Mystical Page will cost: 18k * 450 = 8,100,000
*Guaranteed that you get the skillbook that you want
*Token cost: 18k each
*50 Mysterious Pages = 450 tokens
*Each skillbook from Mysterious Page will cost: 18k * 450 = 8,100,000
*Random skillbook obtained
*20 Old Book Page = 10 mil
*Each skillbook from OBP will cost: 10 mil
*Random skillbook obtained
*30 Page of Fate = 15 mil
*Each skillbook from PoF will cost: 15 mil
*Random skillbook obtained
Some skillbooks are almost non-existant (or very rare) and sometimes, they give a tremendous sage/demon effects that it's almost impossible to get at a same or lower price as if you craft them from *random-obtained* skillbooks. The following books are as follows (don't expect to find the following books for cheap, consider yourself lucky if you find them for cheap):
Archer -Sage/Demon Barrage of Arrows --> PoF only
-Demon Quickshot --> FB 99 drops and Mysterious Pages
Assassin-Sage/Demon Shadow Walk --> FB 99 drops only
-Sage Subsea Strike --> PoF only
-Sage/Demon Twin Strike --> FB 99 drops only
-Sage/Demon Puncture Wound --> FB 99 drops only
Barbarian-Sage/Demon Beast's King Inspiration --> FB 99 drops and Mysterious Pages
-Sage/Demon Penetrate Armor --> OBP only
-Sage/Demon Armageddon --> PoF only
-Sage/Demon Strength of Titans --> PoF only
Blademaster-Sage/Demon Aura of the Golden Bell --> FB 99 drops and Mysterious Pages
-Sage Diamond Sutra --> OBP only
-Sage/Demon Heaven's Flame --> PoF only
Cleric-Sage/Demon Ironheart Blessing --> FB 99 drops and Mysterious Pages
-Sage/Demon Revive --> FB 99 drops and Mysterious Pages
-Sage/Demon Tempest --> PoF only
-Sage/Demon Celestial Guardian Seal --> OBP only
Mystic-Sage Break in the Clouds --> Mysterious Pages only
-Sage/Demon Verdant Shell --> OBP only
-Sage/Demon Cragglord --> PoF only
-Sage/Demon Thicket --> PoF only
Psychic-Sage/Demon Aqua Impact --> FB 99 drops only
-Sage/Demon Spirit Blast --> FB 99 drops only
-Sage/Demon Soul of Vengeance --> Mysterious Pages only
-Sage Bubble of Life --> Mysterious Pages only
-Sage/Demon Sandburst Blast --> OBP only
-Sage/Demon Earth Vector --> PoF only
-Sage/Demon Tide Spirit --> PoF only
Seeker-Sage Blade and Sword Mastery --> OBP only
-Sage/Demon Edged Blur --> PoF only
-Sage/Demon Gemini Slash --> PoF only
-Sage/Demon Yataghan Vortex --> PoF only
Venomancer-Sage Amplify Damage --> FB 99 drops and Mysterious Pages
-Sage/Demon Purge --> FB 99 drops and Mysterious Pages
-Sage/Demon Malefic Crush --> PoF only
-Sage/Demon Parasitic Nova --> PoF only
Wizard-Sage/Demon Stone Barrier --> FB 99 drops and Mysterious Pages
-Sage/Demon The Dragon's Breath --> Mysterious Pages and OBP
-Sage/Demon Blade Tempest --> PoF only
-Sage/Demon Black Ice Dragon Strike --> PoF only
-Sage/Demon Mountain's Seize --> PoF only
Basically, it's the skills that make people go WOW! I wanna be sage/demon cause of that and that skills. When in fact, you should also consider the cost of the books and the rarity of them. There's probably more overpriced skillbooks that I don't know of.