Road tax depends heavily on the vehicle. I think we pay ~€1000 a year, ~€83 a month, for our Hyundai ix35.
* Electric Cars are exempt from both IIRC.
* Diesel is mostly front loaded in the yearly road tax. With a very low tax on fuel. So if you drive a lot it is worth getting a Diesel for this reason alone.
* Petrol has a lighter yearly tax. There is a formula someplace and it has to do with engine size, weight, age and some other things assuming I understood correctly. The yearly tax is cheaper on average than the diesel. Then there is a large tax on petrol. About €0.17 a liter or €0.642 a gallon.