It depends where you go, who owns the airport, the type of airfield etc. For example, Shobdon is FIS at weekends, their pay is very basic as people seem to do it for a hobby (from the flying club environment), where as an airport like Wolverhampton use to be full time FIS and was owned by an airport operator not a flying club -the pay is on a higher scale.
I wouldnt look at a career being a FISO, that's usually done in the the Air Traffic Control Officer position, but it's there for people who are interested, retired, got a lot of spare time and work part time with good pay being a bonus (in my honest opinion). Don't get me wrong, the position is very demanding and i know that from first hand experience.... i wouldn't do it for free ;-)
All the best with your plans,
Ascot