I'd recommend that you first concentrate on trying to convert traffic from free traffic sources before you move on to paid traffic. I'm saying this is because asking how much it costs to buy traffic is the wrong question.
You should be asking what is the cost of converting a subscriber to a paying customer. If you don't have that figure already, you won't be able to tell when you're overpaying for traffic (i.e. the CPC is higher than it should be).
If you want to get started with free traffic methods, try posting videos to YouTube, posting on user forums related to your niche, creating articles to post as guest posts on sites that will accept them, etc.
Use these opportunities to figure out the highest converting landing pages and emails for your offers.
All of these free traffic methods will give you an idea of whether you can convert your offer and how much it really costs to get a paying customer.