And the winner is:

https://www.modmypi.com/pi-modules-upis-advanced?filter_name=UPiS

Because it also charges the batteries if they are drained because of a power fail. Other models doesn't show this feature, I don't know if they have it already.

The model is shipped with the batteries, so despite the price it isn't a rip off, the batteries can high the price of something like this.

https://www.pi-supply.com/product/pi-ups-uninterrupted-power-supply-raspberry-pi/

Pi UPS

Pros: 6AA batteries, so the raspi can run in a powerless scenario for a long time.

Pending: does this device charge the batteries when power returns?

do it yourself:

http://www.repairhub.co.uk/content/resources/raspberry-pi-battery-backup