The email I got below. I did a free trial when it first came out, so I'm not sure if it's only for people on their database.
Effective November 16, 2017, we increased our monthly Zwift membership from USD$9.99 to USD$14.99 (your price may vary due to local currency or tax requirements). Good news, as a valued member of the Zwift community, you’re grandfathered at the old pricing until November 15, 2018 if you join Zwift before December 15, 2017. In other words, you’ve got a month to lock in this lower rate. Click here to learn more.