Advertising on social media can be quite cost effective, but don't expect it to be working as classical advertising! Careful consideration has to be done on what to advertise - e. g. advertising NREN Facebook page itself could perhaps not the best thing to do.
You should reserve about 500-1000 EUR for your campaign or. at least 10 EUR per day. Try various different advertising setups and use the one that performs at it's best.
If you are not sure whether to advertise or not here is a further reading for you: