Envato’s ThemeForest marketplace has added multi-project licensing for their products.
Buy a 10 Project License for just 3 x the cost of a Single Use License.
A 10 project license means you can use the same file over and over – up to ten times – without having to buy it again and again.
They also offer a Web Service Licence, which costs 50x the cost of a Single Use License. The license could be used on a site such as WordPress.com, if Automattic had the inclination to purchase a theme from ThemeForest. A site where the design might be in use by multiple clients, but hosted by yourself.
There doesn’t seem to be a way to upgrade a license you already purchased, but you can simply buy another instance, as people have done in the past when they wanted to use a theme on another site.