As a plastering contractor, it depends on the cracks, and stability of the plaster walls and ceilings.
Pictures would be nice...
There are two problems here...
1) cost
2) stability...
Problem one has been addressed... to a point... first let me explain the difference between Real Plaster repairs, and wannabe plaster repairs...
With real plaster, there is not sanding. period. I dont care who you are or what you claim, your a fake, and you hired a fake if a sanding sponge comes anywhere near a real plaster repair.
Second, real plaster is mixed on site. It doesnt come in a bucket. the lime is often mixed offsite, in a bucket... unless you run out (I have never run out of lime on a job) and then the lime and Plaster of Paris is mixed on site. The reason for this, is even if you split your gauge, your still only talking 30 minutes before everything goes rock hard. You cant slow this down. you mix lime with plaster of paris to get Plaster.
You use a trowel, and a hawk, to apply, and finish plaster. if their using a drywall knife and pan, its not plaster.
You work plaster till it goes rock hard, and then some... to get the glass finish.
The only way to sand plaster is with 40 grit, and a hatchet.
Now that we have removed the wannabe (there are ALOT of wannabes - infact most painters claim to be plasterers yet wouldnt even amount to a pimple on a plasterers ***)
next up is stability...
this takes a trained eye. What am I talking about? I am talking about if the keys are broke... what are keys? Keys are the base coat plaster that is pushed in between the wood lath when the base coat is applied. if those keys are broken, you will get deflection.
heres the problem: Drywall, does NOT have the structural integrity to hold up a plaster ceiling that has most/all of its keys broken... Read: Dangerous Read: if the cieling is not structurally sound, said ceiling could end up in your basement...
Since you have not provided pictures...
assume the worst.
with that said, using an impact gun, and 2" drywall screws, every 6-8" 16 oc will stabilize most ceilings.
however...
If you have spider cracks (most likely) these are cracks that affect the finish coat of plaster, these can be solved by skim coating the walls with drywall compound which is alot cheaper, and easier....
Also, Plaster has many benefits over drywall: It doesnt burn, at all... adds insulation value above and beyond drywall, and is an excellent sound barrier...
Again, pictures would help...
if you cant get pictures, you need to look for a NON PAINTER plastering contractor... as 99% of your painters, claim to be plasterers, but are not... they use drywall compound...and call it plaster.