There are at least two contenders for the title of "Most expensive Rubik's Cube".
The single most expensive Cube, the Masterpiece Cube, was created in 1995, for the 15th anniversary of the Cube attaining international popularity, by Diamond Cutters International. It is estimated to be worth US$2.5 million (2016 value). It is part of the "Beyond Rubik's Cube" exhibition. It is made with 25 gems on each "sticker".
Until 2 December 2017, the most expensive Cube in regular manufacture was the "Over the Top" (17×17×17) cube. To obtain one you need to purchase "Part 1 of 7" (July 2017 price US$275.90) plus six of "Part 2 of 7" (July 2017 price US$395), for a total cost of US$2645.90; but as the cube is shown without stickers, leaving the edges and corners black in the illustration (or white in the purchased version), you probably also need a set of stickers from Olivér's (July 2017 price €8, or US$9.39), increasing the total price to US$2655.29. Alternatively, you could order a ready-assembled and stickered one from Oskar Van Deventer, price unknown.
A more modest alternative is the YJ MoYu 13×13×13 cube, available for a mere £192.96 (US$253.47) (September 2017 price). There has been announced a 17x17x17, the latest in Yuxin's HuangLong series, but as of 18 December 2017, availability is limited.