Thanks for the suggestion! We have 2 different products, the second one being the developer toolkit (http://linkurio.us/toolkit/) so we ended up displaying the pricing on each different page... but we're still unsure about the clearest approach :/
After creating Gephi 8 years ago I'm still a big fan of open source as a production model, but definitely not as an economic model. Companies need support and high-end features to speed up their projects and to limit risks; this is what our tookit is about, adding more than 30 plugins.
Notice that we are happy to give back to the core when the original authors agree. Say thanks to us every time you click on an edge! :)