Relevant section of the license agreement you'll agree to upon downloading:
Subject to the terms and conditions of this Agreement and the payment by You of the applicable fee for the Software, XenForo Limited grants You a limited, non-exclusive, worldwide license to install, download and use a single instance of the Software ("License") through a single installation. Each License may run one instance of the Software, accessible via one URL. Any attempt to circumvent this limitation is prohibited and will result in the License being revoked.
Before deploying the Software, you must register in the XenForo.com customer area the address (URL) at which You will use the License. This URL must be kept up-to-date. You may create a single additional test installation for the purpose of testing the Software. Any test installation of this kind must be password protected, and access to it must be limited to You and Your website staff.
If you can't really afford it right now -but may want to transfer them one at a time as funds allow over - I'd give serious consideration to myBB. There is now a native importer for it.
That's what I'm running 2 of my 5 forums on until I see if they start taking off or not.