Is the 100 credits toll per hero or global? 100 per hero seems a lot (but it costs 3 influence to imperial), instead 100 global feels weak (and I'm not sure it scales well from 4 to 2 rebel players).

It is 100 global, but the card remains in play even if heroes pay the fee. They have to decide each mission if they want to pay 100 or not. So it will be nearby useless in the later part of a campaign, but very strong when achieving it early.