If you add one of the $9.99 older figures that are part of the B1G4 Free to your cart, then add an Imaginators character or even playset, it will show as $0.00. You can do this for up to 4. So, for an older $9.99 character you could in theory get 4 playsets free.
I don't know if this is a bug in their software, or some crazy unadvertised sale to get rid of product, but if it's the former I don't expect it to be active long.
This is confirmed working on the mobile site and using Internet Explorer, but for whatever reason my Firefox still shows the old site and it doesn't work on there.