Two things come into play.
First, what is the reputation of the dealer? If you are purchasing from a reputable dealer they should go through everything on the coach prior to your PDI. A less than reputable dealer will just put it on the lot and use it as a walk through unit until the day of the PDI and let you find the bugs.
Second, what is your experience with RVs? If you have hadd one or two RVs in the past (excluding popup campers) you may have the experience to comfortably operate and examine all of the systems. If not, then an inspector is a good idea.
As for inspectors, in my neck of the woods a good inspector runs about 700.00. The question with inspectors is what do they do for the money you pay them? With a price ranging from 300 to 1200 it sounds like there will be a drastic difference in the service they provide. The cheaper may simply look at major systems, give you a report and say good-bye. The more expensive one may look at all areas of the coach, educate you on how specific systems work, advise you on items to watch in the future, give you a report and tell you to call if you have any further questions. Also, look at the inspector's guarantee. Their guarantee related to service says a lot about their confidence in the work they perform.
Also, see if there are RV mobile techs in your area. Some of them also perform inspections but are not listed as inspectors.
US Army (Ret)
2015 Thor Challenger 37ND
FMCA - F432054