My impression is that the unlearning cost increases as you move from act to act.
I ran through my recent savegame history... it appears that it is based solely on character level. I checked Act #, # of skill points used, # of skills, # of prior skills unlearned. I'm not sure if difficulty level has any effect. At level 1 it costs 33 to unlearn. At 14 it has moved to about 6533. These were the samples I took:
Act 1
1(lvl) - 33(cost to unlearn)
9 - 2700
10 - 3333
Act 2
11 - 4033
12 - 4800
13 - 5633
14 - 6533
Obviously I'm not very far into game so for all I know future acts my make a difference.
