Danny, I've always thought some sort of membership fee for using PPRuNe would be fair given the huge value this site is to us. Not sure the best way of structuring it but maybe along the lines of a 1 month trial membership (free) followed by a paid up membership of say between 20 and 80 Euros per annum. For infrequent users perhaps the option to pay nothing but have more adverts (if they really do any good + limited login time).
Obviously one could come up with all sort of complex ways of calculating members' dues based on individual contribution etc but the above may be an easy way of starting it off.
Best Regards
Rich