Errrr, huge fees?
I don't know what your current indie is, but KGS's fees are not huge in comparison to all the indies that are roughly in the same catchment:
KGS £5,535
G&L £6,060
Notting Hill and Ealing £4,250
Latymer Upper £5,770
WHS/Putney High £5,250/5,290
LEH £5,800
I am not even putting SPGS here - that's where you could say huge fees
So if anything, KGS is at the lower end or middle. I think in London, there isn't really much difference in fees for girls or co-ed schools. The two St Paul's and Westminster are different.
So TBH I am surprised by your quoting fees as an issue for KGS specifically. You may feel underwhelmed, but I think you need to pinpoint more precisely what it is that you were seeking and didn't get from the Open Day vs your current indie.
FWIW, DD got a scholarship to KGS and we turned it down. I had doubts on how well they teach humanities and sports fields that were not on site. On the other hand, I though Maths was absolutely super as well as the sciences. The hockey (that was important to DD) was also excellent.
On of MNers with DD there reassured me re humanities.
I also was not 100% sure on the new Headmaster as Sarah Fletcher was so charismatic and a hard act to follow, but apparently he is quite good and very much of the same school of thought.
All in all we loved KGS, but logistically it was worse than the co-ed we went with in the end. So it was really a choice between locations. The current co-ed runs a bus almost from our doorstep and we liked it very much as well... I actually think maths and hockey are better at KGS than at our current school, but can't have it all.