It depends on the ransomware. We store our backups in a VBK format. If the ransomware actively targets that extension, then yes, the backup files itself can be infected.
There is no difference between the paid and free version. When you want to make sure that the backup files are not infected, you need to implement security on the file share so that the ransomware cannot use the current account from the VMs to browse the network. That's why you should look into a dedicated account to access the network share