Our house came with some furnishings, including a waterbed frame with integrated nightstands and a matching chest of drawers in the MBR. Years prior we had done the waterbed thing already and it wasn’t for us, but we liked the furniture so we just bought a king mattress and set it on the waterbed frame.
After about 15 years we decided to replace the mattress in the MBR and after sticker shock, decided to try an Ikea mattress, only to discover that Ikea king mattresses are not California Kings, so we had to send it back. Ended up getting a mattress from some shop on Morena. We didn’t need a box spring, just the mattress. It’s a nice one and we paid about $800 as I recall.
In the guest bedroom we got a queen mattress and box spring from Ikea. It’s the second-from-the-bottom Ikea mattress and it was $299. I liked the mattress so much I bought a second one to put in the RV. That bed is great – I think I like it better than the one in the MBR.
If you find one you like, Ikea mattresses offer a pretty good bang for the buck.