The cost only depends on your character level, and not the number of skill points put into the skill;
Level [color:"#3A495A"]0[/color]2 = 266 gold [color:"#3A495A"]0[/color]
Level [color:"#3A495A"]0[/color]3 = 600
Level [color:"#3A495A"]0[/color]4 = 1066
Level 30 = 60,000 [color:"#3A495A"]......[/color] (assuming the same pattern holds)
Level 51 = 173,400
