What's your opinion on expensive vs cheap Monitors / Headphones? Do you use different speakers for Mixing and Producing?

I read an interview with a Producer, who is not a mixer, they use Genelec 8030's and said "They are just smooth enough to produce easily on, not too detailed or analytical." And I found out they also use a pretty cheap headphone but well known called K240

My first headphone was K240 and speaker set was FOSTEX
I remember using them for years and I enjoyed them, they werent very expensive, and my old mixes were pretty good

but I thought I needed more HQ monitors / headphones to be professional

so I spent years buyign different sets K712 > Slate VSX > HD800
and speakers Yamaha HS8 > Focal > Genelec etc

but if I had to think, back with K240 and Fostex I was pretty satisfied, I don't think I needed to upgrade.

Professional MIx / Mastering studios usually have many different sets to check on from Low resolution to High resolution

I guess my question is just those in the title, what's your opinion on cheaper monitor/headphones? Are they good enough for mixing and mastering? Are "enjoyable" monitors better for producing? Do you use different sets for producing / mixing or the same?