It's free for the user to download and to use.
The developers put a lot of "expensive" work into it, though, and by means of having to survive to also keep the software alive, they have to find a source of income.
So if you, in some time make lots of money with the shop, or by making the shop for a customer, it's a brilliant idea to go for a supporter membership.
Google will probably help you with the link via: supporter membership virtuemart.
Yes, and it's not me who gets the benefits, so I don't write to fill my own pockets. ;-)
Stefan