It is rather pricey. They do have some 'Gallery' seats at £45 (plus all the usual rip-off charges), but they are either sold out or not on sale yet. I'm not sure what the Gallery is, but TheatreMonkey says
"Mostly sold during the promenade concert season, these are standing places in a gallery that encircles the hall high above and behind the balcony. The atmosphere her is terrific, the sound average and the view vertigo inducing. Again, a unique RAH experience to try just once."
I think £100 is fairly standard for the non-stadium venues for artists that are doing come-back tours. Dolly Parton was the same.