Maybe can be a easy and basic rule, but i have the feeling of being using it badly.
I know how determinate a difficult terrain and their cost in movement in normal and diagonal, but I think it will not apply when the PC have to pay this cost.
the PC pay the movement costs when enter to difficult terrain square from normal square?
I know to move from difficult terrain square to difficult terrain square, you pay the cost.
and when exit from difficult terrain square to normal square the PC must pay the cost movement.
The combat rules have an example of moving into a difficult terrain square from a normal terrain square costing double. The example doesn't cover moving from a difficult terrain square to a normal square, though.
I'm fairly certain that moving from a difficult terrain square into a normal terrain square costs the normal amount, not double, on the principle that a single square of difficult terrain should only hamper you once.