It depends what you want to backup really, is it config? vmdk's? entire lab?
If all you want to do is be able to return it to a usable state should anything happen to your environment then the simplest and cheapest backup would be to simply copy the Virtual Machines folder structure to another drive (preferably external) whilst all machines are powered off, rinse and repeat that every week or so and you know that you have a backup that's at most a week old.
If you want to go a little deeper then you could always look at Veeam, VMware Data Protection or any other VMware ready backup product.
Personally speaking I just make backups of my VM's and keep them on a different storage device because the environment gets refreshed every couple of months anyway.