Ali has what i think are grey market shimano brakes for pretty good prices. MT420 sets around $130nzd and MT520 calipers are $115nzd to me. Haven't seen any mt520 sets but the levers are available for around $100nzd.
Where are you getting your prices?
I agree that they are likely grey market - intended to be assembled on complete bikes.
The levers/calipers that come separately are pre-bled - a barb in the lever pierces the sealed hose, you can just plug and play if the length is right.
My prices were based on my order history, buying in winter here and with coupon sales typically. I usually just find a new store each time, usually the prices have changed by the time I order again, that so that what was the cheapest option is no longer.
These were 85$ cad shipped with a coupon when I bought them a month or so ago. 100$ cad now.
https://a.aliexpress.com/_m0Ksxhf