It’s Anna from Balsamiq Sales and I’m happy to help answer all your questions.
I hope you don’t mind me going in order, so that way I won’t miss any of your questions.
Both products are very similar. The main difference is that Mockups 3 for Google Drive stores your project files in Google Drive and has real-time collaboration available. That being said, we have just released a brand new version: Balsamiq Wireframes for Google Drive! This one is native (no more Flash needed), faster and has a lot of new features you might like
Yes you can and it’s all explained here: https://docs.balsamiq.com/google-drive/wireframes/intro/#importing-from-other-versions
At the moment we haven’t reached a decision on how much version 4 will cost. There could be a small cost involved in upgrading to the new version and any updates, as soon as we have them, will be available on our website: https://balsamiq.com/
That’s correct: https://support.balsamiq.com/sales/updatedesktop/
For Balsamiq Wireframes for Google Drive, the same principale is applied as in the older version: all maintenance and updates are included in the monthly or yearly subscription.
We’d love you to try out the product and you can start a 30 day trial from here: https://balsamiq.com/buy/#gd
Have a look at my first reply as it should also apply to this question.
I hope this covers everything but if you need anything else, you can also reach us at firstname.lastname@example.org