Andrew Boon
As it is now - you can try or even use Dolphin for free. The fee is minimal and one is to pay it only if they've tested the product and perhaps started making money, or just want to look cool without BoonEx banners.

Completely free, and, say, GPL way is possible, theoretically, but it would require much higher scale to be sustainable. I am talking about 3-4x downloads and installations to cover costs. Trust me, it's not a super-profitable operation :) . Wordpress makes money from their hosted see more service (WP VIP), Joomla from upselling demo installations, etc.

As for the liberty to modify - you may modify and resell, but need to keep Boonex references the way we specify - that's our footers. If you want your clients to remove them - $99 per license, paid to BoonEx.
Below is the legacy version of the Boonex site, maintained for Dolphin.Pro 7.x support.
The new Dolphin solution is powered by UNA Community Management System.