The calculation for SNS is a bit complex. MailBluster uses the HTTP/s Endpoint notification delivery type, which costs $0 for the first 100,000 requests a month, and an additional $0.60 per million requests. More details on SNS pricing can be found here.
For example,
To give you an approximate idea of the cost: If you send 100k emails with a 20% open rate and a 10% click rate, the total SNS requests will be (100,000 for delivery + 20,000 for opens + 10,000 clicks) = 130,000. The Amazon SNS charge will be (130,000 – 100,000) = 30,000 x ($0.60/1,000,000) = $0.018.