I think - and admittedly im basing this on nothing more than what i think- she's more about the optics of headlining and being the big name on the poster, and while you can try and swing it as headlining a stage (was it Lana Del Ray who lost her sh*t at not being deferred to enough on the poster? i'm a headliner too, worship meeeee!) if she's competing against some other big names, she might not think its enough of a thing to do for what may well be under her usual payday. There's the whole BBC coverage on free to air tv and renewed interest angle too . . . .
From a crude inflation calc I'll punt £398.
Expensive for folk especially if booking as a family but totally agree that it is (relatively) exceptional value given what it gives access to.