Patching and Plastering - How much will plasterer charge?

I am planning to add on to our 60 year old house next summer and I'd like to blend the new and old spaces as much as possible. We have plaster over gyp-rock (precurser to drywall), no lathe involved. I wonder how much I should expect to pay someone to plaster the walls to match the old ones, including some neat texture that's kind of like a random wrinkling on the wall, don't know the name.

If I do get a plasterer, then how good will my drywall finishing have to be. I was hoping I'd save money by drywalling/taping myself, even though it wouldn't look great, because the plaster will cover it mostly anyway. I guess it depends on how thick the plaster is.

Any thoughts are appreciated. Thanks.

All depends on what part of the country you're and if there are actually any plasterers still working in your area, but I'd think it would be considerably higher than for a drywaller.

I suggest you match the existing with new drywall not plaster. It will be a lot more cost effective and will look just as good especially since you want texture over it. There is no way you will be able to hang and tape then have a plaster do veneer over it. They hang blue board then tape then veneer it.