I would like to ask if the license is one time fee or per year.
Virtualmin is a subscription-based licensing model that renews annually at a substantially discounted rate after the first year. If you don't like the subscription model, you can think of it as upgrade pricing, since you'll be getting new versions of the product constantly via the software repository.
Note that our license server will not trigger a shutdown of service from Virtualmin at the end of the year, and all domains will keep working. But updates will no longer be available for any software provided by us (which is quite a large stack of applications--Apache, PHP5 or PHP4, Dovecot on some platforms, ProFTPd, etc.). We strongly encourage you to keep an active subscription for your server, as it is quite risky to operate a networked server without updates being maintained.
I should make clear that renewals are much cheaper than purchase price, though we haven't actually settled on official pricing yet. It will probably be 33% of initial standard purchase price (excluding discounts, etc.), so if you plan to upgrade more than once every three years you'll come out cheaper.
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As an extension of the original question...
Having once purchased a license, does eligibility for renewal pricing expire with or any time after the expiration of that license?
For example, If I let my license expire and then 6 months later decide to renew, may I do so for the then current renewal price, or must I purchase a new license?
Renewal pricing is only going to be available on non-expired licenses (plus a grace period--probably 30 days). After that, a new license will be required at the regular price.
We won't actually know what the renewal pricing will have to be to keep us in food and housing until we've ended the Early Adopter period and started advertising...but I've committed in the past to it being no more than 33% of the initial retail purchase price. It may end up being slightly less.
That said, no one will need to worry about renewals for another year or so, since we're giving all of our Early Adopters a one year renewal for free on the day EA ends. So, you Early Adopters have a vested interest in reporting lots of difficult bugs to keep us from calling EA finished, since it means you get lots of free license time. And our job is to try hard not to introduce new bugs while adding the necessary features and OS support we've committed to, as quickly as possible so we can start making a proper living off of this business. ;-)