everyone has their own arrangements, what ever works.
I tend to find however that whenever someone asks me how much I charge per hour/day or whatever, they usually think that a m2 price is too much, based on nothing but the feeling it’s too expensive, generally forget to supply the correct materials and expect walls full of tiling in 20 mins.
My M2 price is based on how long my (20 years) experience says the job will take, cost of materials and wear and tear to the cutting tools I need, running costs for the van/business generally, my need to pay TAX and NI, cover holidays/sick and save for a pension. Sometime the job goes south and don’t earn as much as I need, sometimes it goes well and we do very nicely thanks.