Here the girls school is way cheaper £1000 plus a term than the mixed school, but the mixed school is better known.
I think the reasons are historical and sexist in a way. Rich parents have always sent their boys to prestigious schools, while often caring less about their girls education.
However, they still wanted their daughters to be educated somewhere, so smaller less prestigious girls schools grew up.
In many areas really good state grammar schools existed for boys (my father, uncle and DH went to really good boys GS), but the girls state grammars were less good (and girls pushed less hard at 11+)
MC parents looking for a 'niace' reasonable education for DDs who didn't have a brilliant GS or didn't quite pass the 11plus were often sent to small private GS (DFIL dotted on DSIL and sent her to such a school rather than a passable girls GS, DM's because she didn't get the 11+ mainly because neither school or her parents really bothered about girls)
I think as more and more prestigious schools have become mixed, small girls schools came to depend far more on families who could just afford private and who did so because they wanted a nice safe protected enviroment for their precious, delicate DDs.
That this attitude persists in 2014 really annoys me, but it does. My feisty DSIL had to argue with FIL to go to her local 6th form, DDs DF would be much better off in
IMO if she'd done the same.