I have been using these sheets on my King bed for a number of years - very comfortable and well made in my experience, hold together well.
I dont know anyone that has bought them new at 300$ a set tho. This is as cheap as I have seen them. usually 10ish dollars postage. I picked up two sets and that seemed to make postage free.
If a new customer you can usually get a discount code also when you signup.
Dont forget trying for cashback if your with cashrewards (8.4%) like I did.