Personally I would not mind paying for a tool like this if it is good.  I would suggest a one time fee per version, rather than a monthly subscription.  When it gets updated to a new version, you must pay upgrade fee for the new version.  My reason is that I often get busy and can't trade/research for a while, so monthly subs seem like a waste of money sometimes.

Of course free/donation is unbeatable but a paid version might make it easier to fund development.

Can't wait to check this out when I get a chance