This has changed. I noticed today on PTR that Legacy Loot was enabled, so I checked out Highmaul.
I do indeed get 5 items per boss from Highmaul on PTR, but the vendor values are 37% - a bit more than 1/3 - of the values on Live. It seems entirely possible to me that Blizzard will not change the values from what we see on PTR when Shadowlands drops, so anyone farming for gold this way better get it while they can!
A couple of specimen items (Heroic)
Bileslinger’s Censer 36 53 90 → 13 63 63
Vilebreath Mask 32 81 73 → 12 08 43