Heres the thing, Would need to know what year the house was built. as for patching plaster with drywall yeah that's the cheap way, that's how hacks do it

depending on the age of the house you use rock lath or wood lath or metal lath then base coat and skim coat.
But there are other things that add to costs. mold should be treated with Mold Control they sell it at Home Depot.
then what area do you live?
Then I'll tell you what I would charge.
I bid a job next door to where I'm working, the guy had Living room cove ceiling with the plaster coming off the wood lath, then he also had a dining room cove ceiling anf also two bedroom ceilings, I bid the job at $3,000 and some Low Ballers came in at $1,500

These guys show up in Black t shirts and pants looked like a couple of heavy metal rockers

I was watching them and when they seen I was looking they closed the blinds, so I figure the home owner will get what he pays for crap job
