Hi Steve
Yes. It's possible to. For this, you will just need to setup a subscription plan with the following information:
1. Setup fee : 200$
2. Price: 100$
3. Recurring subscription : Yes
Then when users sign up, they will have to pay 300$ (200$ setup fee + 100$ for first month). In the following months, he will only have to pay 100$
Regards,
Tuan