I bought mine 6-ish years ago from IKEA. Double. Cheapest frame and cheapest mattress. I think it was about £300 all in at the time, including a chest if drawers?!
It has served me well but we could definitely do with an upgrade: literally a bigger bed but also a better quality one.
Some prices have almost made my eyes fall out of my head, so I think I need to marinate on the price for a little while to get used to it.
It almost certainly has to be a divan to fit in my shall bedroom.
One bed in John Lewis was £7000... I'm hoping that your king size bed was a bit cheaper than a car?