Graphics card prices/stock have not yet stabilized completely from the last cryptocurrency mining craze.
Thus any low profile 460 or similar is going to overpriced or hard to find, this also inflated the used price of 3xx and 2xx cards.
Also, you have to match the same chipset for crossfire to work, you cant just mix and match say a 360 and a 460, or a 260 and a 270 together.