Dear Davide, it sounds interesting, but in order to answer your question on how much we think to pay, you should spend a post to explain what features do you intend to implement in this pro version...
ps: this is my first post in your forum and I want to say that I'm one of your supporters and I think Sardu is a very useful tool (I also contacted you in the past to give you my compliments!).
I'm rewriting (with a friend) SARDU from scretch. Version 3.x will work cross platform (Windows, Linux and Mac). Will not be compatible with old version, free version will not have the windows section. The linux section will be increased. Commercial version (low cost) will have a license for each computer and will be able to manage more commercial products.
I´ll keep with Paypal donation and if somebody reach a minimal donation established by you -for ex. 30€- he will receive the right for the pro version and updates for 1 year and if he pays 40€ two years and 60€ updates for ever.