Cheap data only sim?

I have a robot lawnmower that has the option to install a sim card so it can maintain a data connection when it's out of WiFi coverage. It will use miniscule amounts of data.

Does anyone know of any physical sim only deals for around €5 a month? Cheapest I can see is around €9 with lyca. Not to be scabby but for the amount of data it's going to use I find it hard to justify any more than a fiver a month. It's not essential, just nice to have.