BizTalk360 on Azure Marketplace is completely a Free offering. You can easily install BizTalk360 through the Marketplace installer (which gets installed on a dedicated VM). However, to use BizTalk360, you need to have a valid license. You can refer to our pricing model here. We have a similar pricing structure for both our on-premise and cloud offering.
You also need to remember that you will need to pay Microsoft for the virtual machine cost, depending on the size of the virtual machine you are provisioning. For more details on the virtual machine pricing, please refer this article published by Microsoft.