"This Tiara was made in 1911 as the centrepiece of Queen Mary's emerald and diamond parure for the Delhi Durbar, which included a necklace (page 174), stomacher, brooch and earrings."
He also quotes from Garrard's ledger for invoices dated 16 June 1911. The cost was five thousand pounds. I presume that included all the individual items. On page 174 he cites a handwritten note by Queen Mary that on 26 May 1911 the King paid for all the mounting and diamonds (except for the marquise stone) in the necklace as a birthday present for her. What Roberts is saying is that the King paid all, even though Queen Mary was originally to pay two thousand pounds.
The diamonds for the tiara came from her dismantled Boucheron tiara and she had purchased the emeralds separately the previous year.