OK
Most people consider that the cost of accommodation is high in Gibraltar, see this site for information
http://propertygibraltar.com/
However many go to live in Spain which is much cheaper, BUT then there are the travelling costs and the time involved and it would mean your son attending a Spanish state school or a private school.
If you live in Gibraltar then it would be simple to get your son into a Gibraltar school which would be free and they teach the English curriculum.
Other costs like food etc are very similar to the UK, you would not need a car in Gibraltar it is very small and the buses are free, although you may like a car so you can explore nearby Spain in your leisure time.
Regards
Grumpy