On a posting yesterday on the Microsoft My Phone forum, a worker from Microsoft confirmed that once the trial for Premium services ends that you can buy the service in full for $4.99.
There is no word on a UK pricing in the thread, but I have posted to ask this to them. The thread is here for anyone interested:
http://social.microsoft.com/Forums/en-US/MyPhone/thread/0a1b9a27-9c8a-433c-9eba-5f064f089f44/
The best bit for me is that this looks to be a one time only price, as my original concern was that this was going to be a monthly subscription service.
The fee will be payable via the Windows Marketplace either on the device or from your My Phone account online.
Let’s jut hope that they can fix the locate my phone feature for the Touch HD:
http://social.microsoft.com/Forums/en-AU/MyPhone/thread/5bf44059-ddb0-45cf-a069-45171293138a