One of the most beautiful tiaras that Queen Elizabeth owns is the one that belonged to GD Vladimir, which was purchased by Queen Mary from Miechen's children after her death in 1920. It is composed of interconnecting circles set with diamonds from which are suspended pendant pearls with small diamond set mounts. Queen Mary composed a matching set of pear shaped cabochon emerald drops which could be worn suspended from the arches of the tiara in place of the pearls; thus the tiara could be worn ensuite with the Cambridge emerald parure The maker, possibly Russian, of this particular piece is unknown; however Cartier seized the opportunity to make copies of it while they were cleaning it for Miechen.
As far as I understand, there were no tiaras amongst Marie Feodorovna's jewels that were sold at her death; this collection comprised what the Dowager Empress considered to be everyday jewellery. However I may be wrong - please feel free to correct me.
Ella's suite of emeralds was sold by Marie Pavlovna the younger to the King of Yugoslavia and for a time adorned his wife "Mignon",second daughter of Marie of Roumania. Queen Marie was the lucky recipient of Miechen's sapphire and diamond tiara, which in turn was inherited by Ileana, Archduchess of Austria, her youngest daughter. Both these jewels were subsequently sold, the emerald tiara surviving in the possession of a well known jeweller, who despite removing the cabochon emeralds, has retained the tiara in its original form.
Finally, as to whether Queen Mary purchased the Dowager Empress' jewels at less than their true value - well you could possibly argue that point. When the jewels were actually sold, the market was depressed due to the large amount of jewellery that had come up for sale and it may be possible that at another time and in another place, the jewellery may have been given a higher valuation. However it has been proven that Queen Mary paid the valuation prices (in full and sometimes over the valuation) and it must be remembered that she bought some of the most valuable pieces. Let's not fool ourselves that she didn't do well out of the sales but ultimately Olga and Xenia did receive what they were owed and Xenia lived until her death in grace and favour residences (at small rents) and was the recipient of a yearly allowance from the British reigning monarchs. Olga, perhaps, was less fortunate; I have never quite understood why her share of the sale was only 40% as opposed to Xenia's 60%...