First, i agree...somewhere in the $400-$700 US range is about normal for just labor. It does take even an experienced installer the better part of a day to perform the install and that is with a "helper" for at least 2-4 hours. At $50 an hour there goes a quick $400 or better and another $100-$200 for his helper, and floor space.
Second, I installed the headliner in my 74 Challenger myself (with the wife helping)...it truly is a job best done by two sets of hands. I watched several youtube videos to get the needed know how. When finished it is "ok" not as good as what a professional with experience would have done. But then those are hard to find for that price... Good enough for a daily driver, but, if it was for a concourse show car i would have gone to a pro. PS...it did take me a full day to do it even with the wife helping.