People apportion their money according to their particular need/wants.
As MTS implies, music is important to her family, so she spends on it. Presumably foregoing what others may consider a luxury (eg cigarettes, nails, tans etc, rounds of golf, football season tickets).
FWIW, here in Bexley (South London) we don't come under the Kent LA. Four grammar schools, Theoretically they take in 25% of the borough children. Or rather, of the average 5,000 Y6 children in Bexley, 1250 grammar places are available. Only 18% of grammar places are taken by Bexley children (so in reality thats 250 children), the rest pour in from inner London, Greenwich in particular. Greenwich has a perfectly respectable comprehensive system, so the one school ethos and streaming clearly doesnt work for parents.
On testing day, it is not unusual to see 700 sets of parents and children queued up out side each of the grammars, all from out of borough. Our own children are done at primary in their own environment the week before.
There is a big parental movement within Bexley to prohibit outsiders!
As well as our own 4 grammar schools, Bexley parents will also enter the Kent tests for Dartford and Wilmington as they are perceived to be 'easier' tests, and you need a back up, the really bright ones will try for St Olaves in Orpington. We are too far away from Judds and Tonbridge.
Even with the comprehensive system in surrounding boroughs, the level of inter-borough transfers is phenomenal - parents do not want a comprehensive system. The sec moderns in Bexley pick up thousands of children, but very few Bexley children move outside unless it is into one of the better Bromley schools.
Strangely there is only one prep school left in Bexley that I know of, but there are many over in Greenwich, all grooming to go across border 